SetCPU's widget takes care of the temperature. Plus you can set a profile to drop the processor speed if you hit a certain temp.SetCPU
TempMonitor (almost a necessity IMHO with SetCPU)
If you don't have a 2.1 based ROM (or one with LiveWallpaper's loaded) doing that manually is nice. I just like the movement of the Wallpaper (I keep the grass one on my Droid)
SetCPU's widget takes care of the temperature. Plus you can set a profile to drop the processor speed if you hit a certain temp.
titaniumbackup is a MUST!