Today, I stumbled across another use that made a huge difference for me. I was using DropStuff to stuff iMovie 2.1.1 (which I had kept around, just in case, when iMovie3 came out). The process was going slowly, especially since I had about 15 other apps open, and DropStuff was in the background. If I moved it to the foreground, it sped up notably, but I wanted to work on other stuff ... so I thought I'd try renice to see what impact it had. In the Terminal, I typed ps ax | grep DropStuff to find out the "process ID" of the DropStuff application:
2295 ?? S 0:00.51 /Applications/Utils/DropStuff 7.0..... 2300 std R+ 0:00.00 grep DropStuffIn order to run renice, you need the process ID, which is the first item output from (in this case, 2295). With that information, I could now renice the process by typing sudo renice -20 2295. Within a second of hitting return (after entering my admin password), the DropStuff operation sped up by an amazing amount -- files were now clicking by at more than two or so per second, and the time estimate dropped by 50%. More importantly, though, iTunes kept chugging right along without a skip, and the system remained responsive, and DropStuff's speed didn't change much at all, regardless of whether it was in the foreground or background.
So if you've got a task lumbering along in the background, try throwing a renice at it. If you don't like the results, just repeat the command with a 0 value instead of -20 to return to the default.
If you prefer to use the GUI over the command line, VersionTracker lists a few candidates. I haven't tried any of those applications, however, so you're on your own...

