I use Diskeeper at work, with the Set and Forget it option on. The program runs like a charm, it's kept my server running smoothly and hassle free since the day it was installed.
I've heard plenty of good things about JK Defrag as well, but I haven't used it personally yet.
I like it because there is no need to install anything, I can use it whenever I feel vista's defragmenter is not doing a good job, another plus is that no resource hog service is running in the background all the time.
I only use JK Defrag. I also like the feature of having it run as a screensaver.
it is pretty good at running for short periods (gets a lot done in a short period of time).
it also moves the unused and junk (uninstall info, etc) files to the slow end of the disk and the smaller more often accessed files to the faster side.
I hav using Diskkeeper for 3year now. I use Dk10 first. Then I upgrade to 2008. It automatic which is nice for me because I am not clever with computers. I see how fast it make my computer. It don't crash so much, espeshily when I watch movies. It was not expensiv but worth it. Use it on Vista. Its the best man!
I got my dad using it in his grocery shop. He has 11 computers and say to me that they are fast and sales go threw very fast now.
JKdefrag is the best i have used, loaded it now, wow my system is much faster. I also love how easy it is, you don;t a million options, it just does its thing.
Flippin awesome program and makes those other pay ones look like rubbish if you ask me.