Rendering...
Rendering, rendering, rendering... lots of Photoshop, Quark, web design, learning new stuff such as Premiere, C4D, Maya, After Effects, but mostly 3D modelling with Strata CX6, and rendering, images and movies. My main client bought it for me about 2 years ago, and the only time it's not running flat out is when we have a power cut! They loved what I do for them, so thought it would be good for them (and me) if I had the best system I could get - which at the time it was. Still stupidly powerful though - I'm hard-pressed to really bring it to it's knees. Quiet too, which means I can have it running constantly without fan noise being a nuisance - some of those PeeCee things sound like they're generating a tornado! It's also meant that the quality of what I do has gone through the roof, as the rendering times are now tolerable, though as has been said, you can never have too much power. Consequently, my client likes me even more, and I got a raft of hard disks and a Wacom Intuos 4 tablet as a bonus! Plus I work from home, come and go as I please, get paid enough to live quite comfortably... life's good!
I actually feel guilty if I'm not using it to do something useful - all that lovely power going to waste, just seems wrong. I never run a screen saver, never play games on it (Use a Mac Pro for playing mere games? Like mowing the lawn with a Lamborghini Diablo!), and every morning, I've got something new to look at. I just love it! Did I mention rendering?