Cels Forum

You all covered Hentai, but there's also Yayoi(gay porn) and who could have guessed, there are several kinds of clean doujin.  

Yayoi-Guys going after guys. Ironically, yayoi stuff is 99.9999%(knowing doujin, there's bound to be one ultra disturbing guy out there to buck the trend) authored by fan girls. There are boatloads of lesbian porn(Utena anyone?)but as far as I know, yayoi is specifically male gay porn. 
I'm sure there are a boatload of people who could confirm and deny that claim.

4 coma/koma-AKA 4 panel.  Most of the 4 panel stuff I've seen has been clean, and gag manga. Although some of the eva 4coma stuff I've seen has been ultra obscure jokes that only the doujin circle that puts it out gets....

Parody stuff-The title explains itself.

Gaiden/Side Story-Doujin can include novel segments. These stories tend to focus on characters that are sometimes neglected or try and explain plot holes in the original story.  

Original-Doujin technically means self published. Some people make their own characters, stories, etc.  

Pro doujin-Sometimes professionals still publish their own doujinshi.  It seems like pro stuff consists of misc sketches, rough drafts for character designs they did, manga done when they have time(more like if they have time) etc.  Pro stuff seems to be highly collectible.