I am just using the stock windows one right now, but have used
http://www.defraggler.com/
in the past and it worked well. I have read articles that said it doesn't matter too much which you use as long as you use something. The reviews said the difference is only in tiny % points.
One reason a lot of people go with an aftermarket is they can be quicker to run than the stock windows, and they can offer schdualing so you can set it up to do in the middle of the night or something.
But the defrag in Vista does that pretty well and is easy to set up a set time.
See here:
CyberNotes: How To Schedule A Drive To Defragment
Microsoft Automatically Schedules Defragmentation In Vista
One other suggestion is no matter what you use, run this program 1st
CyberNotes: WinDirStat Shows What’s Hoggin’ Your Drive
Or the very simlular Sequoiaview (though their site is down for maintainance right now)
http://w3.tue.nl/maintenance/index.html
You can still download here:
http://www.win.tue.nl/cgi-bin/usr/se...counter3xp.cgi
I actually use sequoiaview a couple times a month to clean up all the crap that accidentally gets left around, especially on my OS drive (that I try to keep clean as possible).
I remember the 1st time I used the program I was blown away with what I found and deleted 30 gigs. off an 80 gig drive. I had crap like full length movies that I had backed up but somehow had gotten misplaced (at 4+ gigs apiece), old games that didn't show up in the uninstall list but were still there, etc.
Get rid of all that, then do your defrag.
Funny thing is right now my pagefile is the biggest block on the OS drive at 4 gigs. and it takes up about 20% of the results screen.
Edit:
One more good link I just found:
Defrag on the cheap