I use Defraggler, works pretty well. [s]Think[/s] it works on 64bit too [s]but not sure about that one.[/s]
Although it's not open-source it's still free. And it's made by the people behind CCleaner! ^^

http://www.defraggler.com/
System Requirements

This version will install on PCs running Windows 2000, XP, 2003 and Vista.
64-bit OSs are also supported.