Folding, playing around with fractals and ray tracing, running VMs to try out different Linux distros...
Edit: Just noticed that this thread is in OT&C... TBH I actually don't overclock much these days, but at any given time I do tend to have one or two bare systems I'm messing around with running on the table in my office, so there's almost always a fair bit of "extra power" on tap! Here's what's on the table as of tonight:
On the left: An Athlon II X4 635 running Ubuntu 10.04. This rig is currently my VirtualBox playground; it was supposed to have 8GB of RAM but I got a bad dual-channel kit from Newegg and I'm still waiting for the RMA, so it is currently running on only 4GB. The disks in the cage behind it are a software RAID-1 array (also been using this system to learn how to do bootable software RAID arrays under Linux).
On the right: A Phenom II X2 545 running Ubuntu 10.10. Currently my Ubuntu 10.10 playground (wanted to try 10.10 out native instead of in a VM, to get a better idea of whether I want to switch my main desktop over to it).
Yes, both of them are folding too, with the Linux SMP client...