Giles Turnbull recently spoke to some Mac personalities about their Mac rigs. He found out what Leander Kahney of the Cult of Mac uses, Jonathan
Rentzsh (he of the Red Shed), Michael Tsai (SpamSieve), Joe Kissel (author), and Andy Ihnakto.
It is worth a read, though sadly I wasn't included. Luckily I can just write about what I use right here!
I have a brand new 20 inch iMac G5 (named Vercingetorix. Yes, named after that Vercingetorix) which is my workhorse machine, and a neglected G4 Powerbook by the name of Ambrose.