Backup servers, Secondary Video Capture/Fraps machine, and Enterainment Center are excellent uses.

My son does a lot of programming. Mostly 3D engine lately. He just started on a Minecraft clone.
Anyway, he is on a mac mini right now. Good machine for just about everything except for what he wants to do with it.
He does do IPhone/IOS development too which is why he has the mini in the first place.

I was going to piece together a simple gaming rig for him. Since he doesn't do much boxing yet he doesn't need to much power right now.