matthewfrey is correct - you cannot repartition free space if it is not a contiguous block. iDefrag is a good choice, Drive Genius is another option. Using Carbon Copy Cloner or SuperDuper to clone the drive out and back is actually the best choice - faster, more efficient, and free. (plus you have a bootable backup, just in case)

asle is also correct in that OS X does not require forced defragmentation of the primary drive under normal operation (unlike Windows). OS X maintains proper fragmentation levels on the boot volume all on its own. In fact, you can make your system run SLOWER by defragging the boot volume!

Repartitioning a drive for Boot Camp is NOT a normal operation, however.

Defragmentation is widely recommended for external volumes used for video data storage - you want large continuous block of free space in order to prevent dropped frames.

The question is, does OS X need a defragmenter. Mac OS9 did. I would say it can help if you have many large files (video etc.) or you are running low on disk space. For a starter Cocktail does a good work at maintenance (http://www.maintain.se/cocktail/index.ph...) although ti does not do defrag. You would need something like iDefrag (http://www.coriolis-systems.com/iDefrag.php). I have tried iDefrag with good experience. But did I notice anything else than prettier defragement graphic bars? Not really, or someone else may have different experience. Take a read here about defragement and OS X: http://reviews.cnet.com/8301-13727_7-10328197-263.html

I have found that Onyx is an excellent app for keeping my Macs in top shape. It's also free, which helps. There are many little tweaks in there that will get your system running as well as it can. Hope it helps.

I ran into the same situation before myself - Bootcamp needs enough contiguous free space in order to partition the drive - I used idefrag, it worked well

I am using Fusion to run Windows as a virtual machine. This may be one option for you - instead of buying a defrag program, buy VMware fusion. you can download a trial version (30 days) that can get you up and running at no cost immediately. If you don't like it you can still go back to your original idea

TechToolPro can defrag I have the "for pay" version, I don't know if the free one you can download from apple does defrag?

If you want to defrag the drive and the cloning is not the thing that you want, then in that case you need to use any of the drive defrag software as it might be the case that defrag not let you defrag the files those are more than 20KB of size, while you are editing videos or images most of the file get a lot of space and these all files are out of Mac utility scope.