Neon looking Costumes
uh.... dunno? Try taking a screenshot of him next time you see him?
"My inner mind has become a reality-cracking overgod. He torments me! Help!"
Some of the Resistance pieces from the Going Rogue: Item pack have the glowing effect.
Gloves, shoulders, pants (nice glowing stripe down the side of leg) that I remember off hand.
The final line of a post by Sweet_Sarah on Liberty
"Together we entered a city of strangers, we made it a city of friends, and we leave it a City of Heroes."

Also, certain combinations of textures and patterns tended to have an almost luminuos appearance, especially during nighttime, even back around launch. For example, pairing the Electron texture with a brightly colored Circuitry pattern tended to look like it was glowing. Still, the new Resistance and mutation pieces do it far better.
Dr. Todt's theme.
i make stuff...
Also, the Bioluminescent costume parts from the Mutant Pack have a glow.
Otherwise, it could be the Resistance pieces included with Going Rogue.
But as otehr posters have pointed out, there are certain other parts or patterns that might simulate a glow.
A teammate of mine has recently discovered a way to make some of his costumes to have an almost neon look to them. Certain parts of his toon's costumes shine bright. He having a great time watching me try and figure out how's he's doing it. I know the bioluminescent costumes offer this effect but you can't change the look of the costume. I think I am up to date on all the game's add-ons as well. So... Can anyone offer up an explanation as to what he's doing?