[ffooky]

OS X is greatly less affected by fragmentation than Windows but if you feel the need to defrag then the pick of the bunch is iDefrag <http://www.versiontracker.com/dyn/moreinfo/macosx/25411>.

IMO, the best method if you have a second drive, firewire capable iPod etc is to clone your drive with Carbon Copy Cloner <http://www.versiontracker.com/dyn/moreinfo/macosx/13260>, reformat the source drive and then restore from the back up. If you do go down this route it's best if the target drive has itself been reformatted or even better is a separate partition that can be wiped after the process is complete.

If workable space is at a premium you can burn any files where permissions will not be an issue to CD/DVD and save space on the target drive for the System files etc.