If you don't have shelfari and you like to read/collect/sniff books, I heartily recommend that you get it! You can keep track of when you read books, how many times you've read them, which are your favorites, how you rate them, which ones you own or would like to own... fun! and I want to see all your libraries so I want you to do it :D
and of course if you don't have last.fm, you are missing out on the best music toy ever, so hurry on over and start tracking your music and creating your own radio! I had to go for a few months without my music library due to travel and computer issues, and my personal radio completely filled the gap for me. It's like pandora only about 834534573457 times better because of 1) unlimited skipping/banning and 2) a fully-blended personal station with suggestions that actually make sense (pandora was crap at predicting what I would like).