I hadn't heard of it either until someone I was talking in comments on AO3 told me about it. It's really a fun place - very active and very interactive. So long as you take a few minutes to learn what tags they use for what (e.g. 99% of the time they use "girlxgirl" instead of "femslash") you can find the fics that interest you or, if you're a writer, get your fics in front of the people will be interested in them.