In some cases you want to defragment~
For example, let's say you have 40GB in free space and you want to partition 35GB to BootCamp; the utility might give you a bunch of errors telling you that the partitioning can't be done, files can't be moved, etc., and the only way you can install Windows is to wipe the drive, reinstall OS X, and then go from there.
Well for that you can either reinstall like they say (which could be tedious since you need to move files back from backup as well), or defragment your drive. Defragmenting your drive doesn't always work, but it worth a try since it's less time consuming overall it does make drive access faster.
So the other replies are correct in that you don't need to defragment in OS X in most normal circumstances, but in some cases defragmenting your drive is useful.
Anyways, I use iDefrag to do it. I have Drive Genius 3 and it also defragments, but iDefrag has a visual which helps knowing when it will be done.