Apple Mac OSX is a BSD based system and as such doesnt really need disk defragmentation. However over time, FREE space may become fragmented and may slow down your drive significantly.Free space fragmentation can become a problem when you start running low on disk space. If you have a small percentage of your disk free — and you frequently use and then free-up disk space with temporary files — you’re likely to have high free space fragmentation. This means the free space is spread all over your hard drive in small amounts here and there — its not all laid out in one contiguous section in one spot.
DRIVE GENIOUS is a fantastic tool.
Drive Genius's most popular feature is disk defrag. It has the ability to optimize the drive, by putting all pieces of the file together that over time have become fragmented and spread in various locations across the drive. It then combines all the empty free space together into one contiguous block. This in turn improves the overall speed and performance of the drive.