probably the most comprehensive list of 20in20 comms: http://community.livejournal.com/mod_20in20/profile - just scroll down, it's just for lj but I'm not sure if there's any other 20in20 comm in dw, and from insanejournal i only know of one also called fandom20in20 - but i'm not on ij so i can't really say if 20in20 are common there or not.