Quickly, given its unix base, Mac OS X will benefit from the routine execution of daily, weekly and monthly "maintenance scripts". That is pretty much all you need. The native file system (HFS+) is more or less self-defragmenting until you get down to very low levels of available disk space... at which point you need more disk, not a defragment!
There are several tools that will run the maintenance scripts for you. My favorite is
OnyX. It is freeware and will do not only the scripts, but lots of other things as well, including allowing you to tinker with some hidden OS settings (think of the old Windows TweakUI and you will have the idea).
I run OnyX about once a month, or whenever I notice that my Mac seems to not be as fast as I would like, and it cleans things up nicely. Good as new once run and then restarted.