An American librarian, Patti, wrote about it on the Fiction_L list. I'm sure she won't mind me showing you what her Good Reads page looks like here. As you can see you don't just record what you have read, you can list what you'd like to read - for me sitting down here in Cataloguing, with all that yummy new fiction coming across my desk, that list gets bigger every day.
There's a networking element to this one too so you can see what your friends are reading. Might send you an email sometime soon?
I'm off to give it a go and will then add another widget gadget.