I always TRY to help out when it looks like a hero's in trouble. I can't count the number of times someone's sent a heal my way while I was fighting. I'm always grateful.
I jumped into a fight one time, but the other guy got killed anyway, so I gave him a revive inspiration. Didn't take anything in return, just wanted to help. My character doesn't really have any assisting powers, so the only way I can really help is to jump the villians and take some aggro. This HAS led to angry accusations of kill-stealing, but I try not to let that dissuade me from helping too much.
I try to check the health and level of the hero and villians before interfering, but I DO sometimes forget and just jump in swinging. Though I can say that I have never yelled at anyone for jumping into my fight. I've been rescued too many times by other heroes to get angry about it.
Because of Hero's anture as a damn dirty communist, he pretty much gives away 10,000's of influence whenever He has a moment to spare. Heh, I always am gratful for any help anyone gives me, be it a heal or an inspiration, or just recently in SS. An excellent Character on Virtue, named Horror Show, took some excellent pics of my team toppling the Babbage. he posted them and it got me excited....yeah, im lame.
Every time I see your avatar, Ripperjack, it makes me laugh!
Kudos!
Anyway, back to the reason I am replying...
When I was first starting out, I received a tell from "Deus Ex." Apparently he had seen some of my posts, and said he had something for me...
Well, the last time someone said that, I was in deep trouble, but I figured: "What the heck, you only die... ummm... 50 million times" (Debt can be quite extreme for a solo controller first starting out...)
So I met up with him. He have me 10,000 influence, right there, on the spot, and left on his merry way.
I believe he said "It's important to help those that show promise."
I mean... AMAZING! He sent me an e-mail later about teaming up... and I replied, but he never got back to me...
I wonder who that masked man was...
I often help whenever I'm able. But what I try to do, is either O2 Boost someone, or flash the enemy... ... ... (wait, that didn't come out right)
ANYWAY, flash blinds the group, but takes no damage, and thus I don't "KS."
However, if I'm in the middle of a pitched battle, alone and soon to fall, I don't care WHO comes along and wipes the floor with my would be murderers.
I mean, even power players need to realize this simple fact...
The DEBT you were about to receive FAR OUTWEIGHS the experience that helping hero just stole!! Lighten up, please...
On a more happy note, when I play a tank I also enjoy provoking all the enemies to attack me, but don't attack them, this way I can let the wounded hero finish them off without me "stealing" anything...
Just a thought to anyone who likes to help, but doesn't want to incur the horrible wrath of the "KS Kryer." 
Good topic, good post, always enjoy when I hear from you. Look me up, or First Strike Security!
~The Sly Fox~
I've done it a few times with different characters.
Playing Creepy Crawly (I think) I helped out a guy getting pretty badly beat on by some Vazh. I called out to him in /local, but the poor guy was too busy fighting. When I saw he was starting to run away (or try to) I took down a couple of the bad guys, then when the dust settled, he thanked me for helping out and apologised that he hadn't said anything - he was too busy fighting. 
Playing as Sky Pirate, if I see a big group and a character being beat on, I'll send them a /tell to ask if they want help. I tend not to interfere unless I'm specifically asked to, or if I get dragged into the fight by accident. I try to make sure I use /em salute afterwards too, given that SP is a patriotic guy.
As I run around Paragon City with one of my powers set to 'default use' (you know, with the green circle around it) I've occasionally shot at a bad guy without meaning too, usually because I've clicked on an interesting looking hero and then 'assisted' him or her by accident. I always make sure to apologise in advance if that happens, but, fingers crossed, haven't been accused of kill-stealing yet.
I guess most of the time it's not a problem because groups of 'mobs' are spaced out pretty well - but in Perez Park last night there were a couple of teams who were overlapping as we went around the zone's periphery. In this case no-one seemed to mind when everyone's mobs got mixed up into a big mess!
I really try my hardest to 'act the hero' in game, but to be honest, I see it as an extension of the way I act in real life. Without trying to paint myself as some sort of saint, I try to be considerate and never forget there's a real person controlling that hero, somewhere.
I play on Champion. A few days ago I was in the red zone in Skyway soloing, and decided to take on a Boss mob that has psionic powers. The battle went badly, because at the time I had not yet aquired psionic defense. He killed me without my having done much damage.
Since I had run a little ways at the last minute, I thought I was far enough away to use an awaken. Nope. When I stood up, disoriented and stumbing around, he came after me again.
At this point, another player named O Ya, who was at least one and maybe 2 levels lower than me, came over and taunted the mob off of me, long enough for me to get away. This was at great risk to himself, since he was in no better position to kill it than I was, and basically took a beating long enough for me to stumble off. He gained nothing, because he had to flee as soon as I was safe.
When I sent him a tell thanking him for the rescue, he responded "that's what heros do." Class act. 
As an epilogue, if you have an an opportunity to get psionic defense, grab it. I have, and have spanked that Boss mob since many times.
When someone complains about drive by heals and such, I crack up. Especially when I'm in Perez, I LOVE when someone just jumps in and starts swinging, healing whatever. I mean, when I jump into a group of 15-17 gang-bangers(ESPECIALLY groups with multiple lieutenants), I think I can probably afford the "lost experience" I get for the thrill of standing back to back and taking out some baddies. Plus , the heals are REALLY nice lol. I mean, I have Regen, with reconstruction, and siphon from dark melee, but sometimes, I get a little excited and don't watch my HP so well. hehehehehehehe
I mean, I had someone following me around last night, a controller, sniping off jokers from the mobs I was jumping, and had no problem with it, til they said something along the lines of "it's fun just leechin" or something. For some reason, that bugged me.(I'm moody) 
And I like trying to help out others, too. Also last night, I dropped an awaken off on a Tanker a few levels below me who had bitten off a bit more than he could chew. I guess I COULD have jumped in the fight and tried to keep him from dying, but I was afraid of the dreaded "KS" tag, so I waited, then when he dropped, I ran off the baddies and gave him the inspiration(which, in hindsight I could have used about an hour later ) and we went and dropped the baddies together, him getting most of the XP and a couple of inspirations as well. He offered me them in exchange for helping him out, but I refused. I figured, what the heck, it was fun.
Also, over on Guardian, Gendarme once came in to help some friends of mine and I who had taken on the Dr Vahz mission a bit to early, and a bit underpowered.(no scrappers, no blasters, lol) two tanks a controller and a defender. no WAY were we getting through Doc's regen. We went to recruit blasters, and added a lvl 10 who just wanted in on Doc, and Gendarme(who we didn't know was lvl 40 when he sent us a reply lol) who got into the mission zone and then dropped himself from the group, so we could get all the experience, and kept Doc's flunkies aggro'd on him so we could deal with the big tard. That was a real cool moment, and a cool guy. That was one of the moments that made me think that helping out folks a few levels under me was a pretty cool thing. I'ma keep doing it, as it also fits my characters bio for RP (hehehe) and if I have to apologize for jumping something someone didn't want me to, I'll do that.
Just gonna have to be careful, cuz I don't want a rep for being TOO jumpy.
Also, on an odd note, I had a guy jump down my throat for trying to GIVE AWAY enhancements the other day. Some people are just strange.
Its been a month now, patrolling the streets of Paragon City, and Im seeing a lot of experience grinding of late. Now Im not dumping one the XP grinders out there, you paid good money for your client software. If you want to do that, then more power to you. Im not going to tell you to do otherwise.
) and headed down to street level. I was lucky enough to get to the pavement and quickly catch up to the hero, by using a few back ally short cuts.
However, I want to hear the stories from the TRUE heroes of Paragon city. The players who will give inspirations to the younger characters, to help them out, or players who will put their characters life on the line, to step in and assist another hero about to be mashed to a pulp by a overpowered group of skulls.
For example, I was patrolling around Kings Row with my leading character. Being a defender I like to stay at range add attack from the roof tops, where its relatively safe. Anyway while I was checking out a potential group of thugs, I spied another hero madly dashing along the road, with a group of outcasts hot on his tail.
Now normally I wouldnt worry about such a site, since I see heros all the time running around the city, but his health bar was but a sliver from defeat, and the group was in hot pursuit. At that point a few thoughts flashed through my head
Should I assist?
If I help, will this be considered kill stealing?
Bugger it! This player was in trouble, Im in a position to help, and even if I got next to no experience for the thugs (who were grey to me) I cant just sit by. If anything, I can always apologise later, and offer a few inspirations to smooth things over.
With that, I kicked in my hover (I know, Im working on getting fly
Stepping out behind the building I immediately threw a desperation bubble over the hero then stood between him and the thugs. With seconds to spare I fired an energy torrent to throw the thugs off the chase, giving myself and other hero time to better prepare for the counter attack.
With volleys of Power Bolts, Power Bursts and a few more Energy Torrents the thugs were quickly defeated.
As the dust settled, I asked the hero if he was OK. He said that he was, and that he was heading back to the trainer. With that I bid him good bye and good luck.
I didnt get much experience, I didnt get any loot, but hey. I felt good that I made prospective new friend and possible ally, and that I was living up to the core sprit of the game.
That of being a hero!
So if youve got a heroic player story where either you help or were helped by another player. Please, post it here. Id be happy to read your exploits and perhapse meet you in game, where we can share the sprit of the game together.
Please note: Dont get me wrong. Im not saying that the NPC heros of the game, such at Statesman, Backstreet Brawler and the others arent heroic, as I dont see them in game saving players when they are in trouble. I am sure, these characters are very heroic, upholding the lofty ideals of law, order and justice.
Just that for me, actions speak louder than words. So if I see another character selflessly helping out a new player, to me they are the real heroes of the game.