Message board fan fics are quite annoying as sometimes, I have to go through several pages skipping hoardes of comments get to the next chapter. Also you have to be signed up to certain message boards just to read them. A lot of these are fanfics, which have been exclusively posted on those message boards and not anywhere else.
FanFiction.Net has thousands of Power Rangers fan fiction spread over 400+ pages.
Paladar's World Of FanFic (
http://www.paladar.com/fanware/index.htm) - This site has some great fan fic for the Zordon-era and Lost Galaxy.
I can't think of any others but an archive of all of them, and other fandoms would be great. But it would need a good search facility otherwise you could be spending nearly an hour or more searching through pages just to look for the fanfic you want to read.