Well, that was one reason. It would be used mainly under Linux, though I really need to start learning the basics of OSX. I've seen quite a few in good cosmetic condition on eBay for less than $100. The main reason I want one is because they're they only way I'll buy an Apple product (used), I want a desktop again just because I want a desktop again, heat isn't an issue (and I'm not paying for electricity), it'd be only lightly used (or possibly not, depends on if I folded with it when not messing with Linux and OSX, playing Diablo II or document editing) and (main reason) because they're gorgeous. I absolutely love the aesthetics of the PowerMac/Mac Pro.
Lonewolf, I'd be happy to take one off of your hands for shipping + a nominal fee. I'm even only slightly joking (but only because I'm guessing you won't accept)!
They belong to my workplace, so it's not a matter of "won't" as much as "am unable to".
Personally, Craigslist would be my first spot. I admit, for every one person with a reasonable price on their used Mac, there are five who think it must be worth 20% more because it's a Mac; while that might be true for the right model, it is not for the G5 tower.
Just helped a colleague get a late model Macbook (Core 2 Penryn, Geforce graphics) off CL for an unheard of $350, because it needs a new inverter cable; the display backlight occasionally turns off. It's going to stink to repair (Apple = design first, engineering for repair = 2nd or 3rd), but with a $15-30 cable, we'll have a laptop worth about $200 more than he paid for it.
Got my dad a 20" aluminum iMac from a business moving to PC about two years ago for $550. I just upgraded it to Snow Leopard (it had Leopard, and I don't want to confuse him with Lion, he's not really a computer guy) this past month. It's all about watching, knowing what you want, and being the first to jump in.
Use everymac.com for specs --it's the best site I know of for that sort of thing.