This week, Lifehacker recommended the top five book recommendation sites.
In the past, I've signed up for Shelfari and LibraryThing, but just found them too tedious to use. I use Amazon almost exclusively and find that the recommendations are usually pretty good. I think published writers (and agents, and editors) should at least belong to all five, just to help get the word out about their books. They're all free, so cost isn't an issue! :)
Update: GoodReads is getting the most votes right now at Lifehacker, so I'm trying it out. It's got a widget for the blog, so I'm trying that out, too.