I'm finally using my old 3500+ again. 130nm architecture single core, hot running, poor overclocking piece of silicon in a stunt rig for my two year old. He loves the thing for at least 15 minutes at a time.
Anyone who has an old chip and some DDR sitting around can make a great server running Windows Home Server. Automatic backups, small footpront and reportedly works flawlessly. Throw an old A64 and 939 board with 512MB of RAM into a beat up old case with a pile of your old hard drives installed, load Windows Home Server software and stick the thing in a closet. It distributes the data onto all the hard drives connected as if they are one big partition and backs up all the PCs on the network automatically.
Unlike Vista this OS has received rave reviews form all the usual suspects. Great way to put all that old hardware back to work. A little pricey at $170 for an OEM copy but hooked up to a 300 watt UPS it could be a real lifesaver and lighten some of the guilt we all suffer from our many upgrades.
http://www.maximumpc.com/article/windows_home_serverhttp://www.newegg.com/Product/Product.aspx...N82E16832116395