Being a longtime Mac user, I've seen this question before. I am quite certain that defagging cannot be done on Macs. If you hear of something to the contrary by all means let me know, but I even checked the Mac OSX reference manual and it didn't mention anything on that subject matter.
>I am quite certain that defagging cannot be done on Macs.
Fragmentation isn't as big a deal as it used to be. With large hard drives, hard drive caching, and more intelligent filesystems, data isn't fragmented in the first place as often. However, defragmenting *can* be done on Macs, in fact, it's done on-the-fly. A feature built into the Darwin kernel called "Hot-File-Adaptive-Clustering" helps keep fragmentation low, automatically.
In the case that fragmentation is more serious, there is a third party tool called iDefrag that can do the job instead. Or, you can simply wipe the disk and reinstall.
TechTool Pro defrags the main HDD and any external I've used since being on Leopard. How much it helps, I'm not sure. It's sort of like Disk Warrior's optimization.
Not sure if it's compatible with 3.9. I still use Disk Warrior on my eMac that has that.