I'll second that recommendation for iDefrag. While it's true that OS X does defrag certain files as mentioned already, and it's not necessary to defrag often like you do in Windows, I can say I've experienced clear performance changes in OS X on occasion. In one case, I had a Powerbook that was taking well upwards of 5 minutes just to boot up. After all other troubleshooting and maintenance tricks failed to fix it, a defrag recovered my boot times back down to a more normal time of a minute or so. There are a couple other times I've benefited from defrags on other Macs, but this is the most drastic example. And a couple times, I really saw no difference.
My general impression is that people who work with a lot of large files, frequently download, save, delete large files, and so on in that vein, will see some performance improvements by defragging every so often. I'd also like to point out that iDefrag also doesn't just defrag the files, but also "optimizes" the file layout to allow for greater efficiency.
