I have to say that I adore this LJ community: What Was That Book?.
It's like a big mystery story just trying to figure out from the little clues people remember about a certain book what exactly it is. And, it makes me realize how few books I've really read when there are so many out there I can't ID!
And, I sometimes find some very interesting sounding books there that I just have to read! Like A Traveller in Time by Alison Uttley.