Life hacker has a write up of the Five Best Disk Defragmenters. I have personally used Diskeeper for about 3-4 years and never had a problem. It will run in the background and you never notice it taking up CPU while you are working. My drives are always running at their optimum speeds. The reading and writing of large files is where you can really notice the difference. http://lifehacker.com/5348638/five-best-disk-defragmenters
Here is a link to a trial version of Diskeeper: http://www.diskeeper.com/diskeeper/home/trialware.aspx
The retail version costs $29.95 you can purchase it here: http://www.diskeeper.com/diskeeper/home/purchase.aspx