Drifting slightly off-topic, but OS X doesn't really need much defragging. The OS itself will defrag an individual fragmented file whenever it's actually opened (subject to certain rules about its size, disk free space & file fragmentation). So a fragmented file that's never opened won't get defragged, but that's not important.
OS X doesn't however squash files up in order to optimise the free space into big chunks, so a 3rd party utility could be of benefit there, but it's only likely to be important if you do lots of video editing etc.
Unless things have changed with Windows7, Windows doesn't even do this much.