I've had a Mac Mini sitting in my garage for a good two years now. It works, to my knowledge, aside from a bad hard drive and no power supply. Last week sometime I ordered a power supply for about $20 on eBay, $10 under the standard price point ;-).
So yesterday was Day 1 of the attempt to turn it into a Media Center or HTPC. Obviously this being a PPC G4 Mini, it won't run anything more than 720p from what I've read. But that doesn't matter, because it's going in the bedroom and the best I can stream over 802.11G is 720p MKV's.
Linux Mint is my ideal OS for home stuff these days, but unfortunately they don't have a PowerPC Version. Ubuntu and Debian do, so I'm going with Ubuntu. I can always download and install the codecs and whatnot later.
The machine only has 512MB PC3200, but my wife's machine has a spare 1GB stick of PC3200, so that's the first thing I swap out.