A few months ago I was able to order a new machine at work, a Dell XPS 670 workstation. It's a pretty serious workstation and in fact, around the office we've been calling these boxes "our mainframes". Mine has two dual core Intel Xeon CPU's with 4GB of RAM and two 250GB 7200 RPM hard drives configured using RAID 0.
Keeping this machine from idling all the time can be a difficult task so I just setup a VMware player running Windows XP SP2 Pro using CruiseControl.NET to run our ASP.NET smoke test. The VM runs minimized in the taskbar and builds whenever the source repository changes. At this point, CruiseControl has been running for 2 days and built and run the smoke test successfully 28 times.
I figure this should help chew up some of these spare cycles