Take a look at LibraryThing and catalog some of your favorite books.
If you're starting to keep a reading log or journal, LibraryThing is a good place to begin.
One cool feature is the ability to edit your book covers. For some reason, seeing what the book looked like helps me remember its content. This feature also lets you give your library that personal touch—you can make your online bookshelf look just like the real one in your living room.
Other applications that track books are Shelfari and GoodReads, both of which are now owned by Amazon. Although Amazon recently purchased a significant share of LibraryThing, subscribers can keep their accounts private. A drawback is that you can only log 200 books for free.