Hey! I'm a member (and admin) of two fanfiction sites. As I'm sure you can guess, they're titled All Star Marvel and Ultimate DC respectively. While neither site is huge, the writers are loyal, hard working and incredibly collaborative. Together, we've created new Marvel and DC Universes in the vein of Marvel's Ultimate line, and we're always looking for new blood.
Ultimate DC is the older of the two, and run by the kind couple DiscipleofBob and adrini. I back them up as the third and final admin.
http://ultimatedc.proboards.com/
All Star Marvel is younger and more active. Additionally, it features a favorite creation of mine--The FanCon. Just check out the site for more details about it. The three admins of ASM are myself (the head), All Star Silentking, and thetrueelec.
http://allstarmarvel.proboards.com/
Feel free to ask me any questions here or on the sites. I'll get to them within 24 hours, unless something serious happens in my day-to-day life that keeps me from answering immediately.
Thanks! I hope you'll check out All Star Marvel and Ultimate DC!