Diskeeper
I downloaded Diskeeper the other day and installed it. I'm pretty impressed with it. It was an easy install and once I set it up, it just runs. I haven't had to mess with it since. It has kept my drive perfect for a while now.
If you want to check it out, you can go here
It works on Vista also, though I only ran it on XP. They also have some extra features if you're a network admin to control and defrag you're entire network.