I downloaded iDefrag 1.6.1 (the latest version I think) from VersionTracker. UPDATE iDefrag is bundled with a special CD creator program that allow you to make a bootable CD or DVD with iDefrag on it. I bought the package and ran it on my Feb 2006 iMac (the first time since I bought it). My HD is not that full - only about 150 GB of the 500 GB total are in use. But it still took many hours to defrag - I started it at about 6 PM last night and it was about half done at midnight so I let it run overnight. But in the end it was well worth it. Everything runs noticeably faster - from boot up to app launch.