I know email groups are so 1998, but I still subscribe to several. Here are some of my favorites and a brief description of each:
robotechsadf- My yahoo group. It is a very loosely formatted Robotech play-by-post game. It never got much further than me and my brother-in-law goofing around with some Robotech characters. But I have kept it alive.
mv1talk - Marvel Volume One The granddaddy of all online Marvel fanfiction. At one time had many different authors. I think the old saying to many cooks in the kitchen kind of took over here. Some of the best Marvel fanfiction on the net. But suffers from poor organization and way too many dead links. Worth digging for the stuff that is still around though.
avengers2000 - AV2K Another giant of the online Marvel fanfiction world. Many good stories here. The different imprints can be rather confusing. I think this site kind of took over title of king of online Marvel fanficiton when MV1 started dying down.
ma-talk - Marvel Anthology - While I'm covering online Marvel fanfiction I can't leave out this one. It's not my favorite, but I do like some of the stories here including the Heroes for Hire series. I can't really recommend anything else because I haven't read many of the stories.