For what OS? XP? there are a couple: Diskkeeper comes to mind, and there's another whos name escapes me at the moment. I know Diskkeeper can run while you use the PC, and I think the other one can as well.
<font color=blue>Outside of a dog, a book is man's best friend. Inside the dog its too dark to read.
-- Groucho Marx</font color=blue>
I use both Diskeeper Pro and PerfectDisk 7.0 ... a software guru who used to post here often recommended both ... so I bought them ... and both allow you to defrag while you do other tasks (and a lot more too).
<b>"The Edge - there is no honest way to explain it because the only people who really know where it is are the ones who have gone over." Hunter S. Thompson, </b><b><i>Hell's Angels, 1965</i></b>