Addiction: A Harry Potter Doujinshi Website
Half of the reason for why I started collecting doujinshi in the first place can be traced back to DrMM, who basically introduced the whole concept of "doujinshi" to me back in the day . . . AND implanted subliminal messages so I'd fall in love with ZoroxNami. This is her Harry Potter doujinshi site, and even though she stopped collecting a while ago I still have a LOOOONG way to go before I reach that kind of collection.
Check out the cute sheep graphic! This site focuses mostly on FMA, Bleach, and Gundam Seed doujinshi, but there are a lot of other fandoms like Ghost Hunt and TURN ? GUNDAM are also represented. Take a look around.
Twinbells collects doujinshi AND sells it too. But if you collect doujinshi at all, you already know that. Her collection/reference site is very nice, the largest section being Naruto. If you like het doujinshi, check it out.