Fanfiction.net - This website consists of one of the largest fanfiction archive on the internet. You can post your own stories, get reviewed, read other's, and review them. The problem with this site is that in the past few years it has banned NC-17 fics and has recently changed its rating system.
MediaMiner - Pretty much the same thing as Fanfiction.net but less censored and not as well known. If you plan on writing stories that are rated NC-17 then click here.
nFiction - A Fanfiction.net clone that allows NC-17 fanfics.
The Dragon Ball Fanfiction Library - Just as the title says, if you're specifically looking for Dragon Ball fanfiction than this is the place to go (although it does have crossovers and other anime is featured).
Neo B-Chan's Homepage - Formerly known as B-Chan's Homepage. B-Chan has some of the most well written, epic, and novel fanfiction that's on the internet. You've got to check it out. She also has a large selection of well done artwork that corresponds to her stories.
Vegeta Insaiyan - This is a great DBZ fanfiction archive provided by Vegeta Goddess, in which you're bound to the fanfiction that satisfies your thirst. She specializes in dark Vegeta fics as well as extreme odd couple fics.
DBZ Romance - "Where love never goes un-noticed." You get the idea, now check out the fanfiction!