ithurtsmybrain is old and hasn't updated in forever, but the prompts that are still there are pretty amazing. The idea was that participants periodically submitted lists of characters from a variety of fandoms, and then the moderators randomly paired them up and challenged people to write fics about those pairings. I think it was intended to be shippy, but I tended to take it as more gen oriented simply because some of the pairings didn't work for me any other way (some did, of course). I haven't actually written any of these, but trying to figure out how some of them could work is a lot fun for me. I'd say that more than 95% of the prompts never got fic.

tumblr_refuge is a meta discussion group for folks who are in fandoms active on Tumblr but who want discussion. All posts are locked by default, but joining is pretty easy. I haven't seen a new post for a while now, but when posts do come up, there're generally a lot of comments. I think the community originated in Sherlock fandom, but it's not limited to that.

whatwasthatbook is a community for people to post about books they know they read once but for which they can't recall the title or author. Some books come up fairly regularly. Some books get identified quickly. Some never do. This is the sort of community that works better with more people reading the entries. I find it fascinating to see what things stick in people's memories about what they've read. Books from the last thirty years are more likely to get identified than older books, but some older books get identified, too.