Here's the official word from Apple on defragmentation. What they say, essentially, is that with OS X there's not much to be gained by doing it unless you do a lot of video editing. If you do decide you need to do it you'll have to use a third-party utility.
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There is another Mac utility called MacJanitor, it cleans up the disc without having to leave your computer on all night.
You can download it here for free.
One more suggestion, especially if you run any Microsoft applications on your mac or do file sharing.
Download Peer Guardian and it will block attempts at spying from government, Edu, Internet providers, Spyware, Microsoft Corp and others.
It too is free. I've had file sharing sessions where as many as 2 thousand hits were made on my computer, all turned away by this free application.
Another exceptionally useful Mac utility is MainMenu. It runs all the routine maintenance scripts that Albert mentions plus lots of other useful tasks like rebuilding your Spotlight index, repairing disk permissions, clearing caches and histories and removing temporary files, all from a single icon in the menu bar.