Well. You have disk utility which repairs permissions and things. Basically if you move a lot of stuff around, delete a lot of stuff or install/uninstall it can be good to repair the permissions, which in my limited understanding is equilivant to defragging.

iDefrag was recommended to me as my 2ghz. dual core 2gb rammed up beast would struggle. It's tidied everything up and is slightly faster I guess.

But, it's not really something I understand. Or attempt to.