Well, there is an 'announce your latest creation' thread up at the top of this forum section...and this whole section is kind of the official knowledge base on arcs. It's just not very organized, I'm afraid. 
A true arc wiki, to my knowledge, does not exist. I don't imagine it ever will, either. MA is just way too large to keep track of everything.
"If I had Force powers, vacuum or not my cape/clothes/hair would always be blowing in the Dramatic Wind." - Tenzhi
Characters
There is also a new fan content wiki at http://ouroportal.com - it's for player created content including MA arcs, character bios, etc. as opposed to the developer created content at paragonwiki.
...I forgot what experience means.
Just wondering if there is a Wiki for StoryArcs. It would be a good place to look up Arcs or post general information about Arcs. If there is such a place, mind giving me directions?