It serves a small office, allowing iMac users to log in to their network account and access their files from any machine, courtesy of OS X Server (which I love, by the way, so impressed), it runs pro apps from Adobe and Macromedia, it runs MS Office applications, Camino, MSN and a load of other stuff - plus, of course, WoW - which is not only fantastic on the Mac Pro with ATI X1900 but made even more droolsome by Apple's 30" ACD.
So, all in all, it's a busy little bee - yet you'd never know it, it's cool (actually cold to the touch!), quiet and reliable, and the dual 2.66GHz 'Woodcrest' Xeons are just the most powerful thing I have ever experienced. This machine makes my Precision 670 with dual 3.6GHz (NetBurst) Xeons look like an OAP that has just been run over repeatedly by a tank.
For me, using a computer will never be the same.