Currently Being ModeratedFeb 19, 2012 7:40 PM (in response to raymondfromhuntersville)
is officially obsolete, it will still give you basic info you're looking for.
Unless you're doing video editing, or you're very low in disk space, defragmenting on Mac OS X is a waste of time and potential danger of data loss (if you get a power outage during the process).
If you're very low in disk space, you might benefit from defragmenting; however, that should be your positively last option. It's much better to clear up space by deleting or archiving files no longer needed, or to increase your storage by replacing your hard disk or getting an external disk. (You might be told that Mac OS X requires 10% or 15% or 20% of your hard disk as free space in order to run well. That's an urban legend.)