LibraryThing

From LibraryThing:

"Enter what you're reading or your whole library—it's an easy, library-quality catalog. LibraryThing also connects you with people who read the same things."

An excellent resource for avid readers everywhere, LibraryThing allows you to catalog up to 200 of your books for free (unlimited books: $10 for a year or $25 for a lifetime). Not only can you catalog your library with ease, but you can find other users with the same taste and interests. LibraryThing creates a great community of readers.