I saw an offer I couldn't refuse on reddit, a PowerPC (PPC) Mac Mini and Logitech mouse and keyboard used for $100. After a week of mulling the idea over, I had to have it.
It came with OS X 10.5.6 which was upgradeable to 10.5.8. However, to get the partition setup I wanted, I had to reformat. This meant going back to 10.4 since I don't have the 10.5 disks. I'm installing Debian 7.7 now, since that's been updated in 2014. The machine only has 512MB of RAM, so I'll be looking to upgrade that. And might throw in an SSD so it doesn't drive me too insane.
Reasons for buying this are getting experience with the PPC architecture (diminishing returns, I realize, with this type of CPU being phased out). Also, getting more experience with OS X, since I've previously only developed for iOS (worked on OS X, but not for it). Why'd I'd want "experience" with PPC is because it's different from x86, different endianness, performance characteristics, etc.. This allows me to test my programs across a wider variety of platforms and architectures.
Who knows if I'll have time in the coming months to work much with it (working full-time on Star Citizen and all), but I'm sure I'll have some fun with it eventually. :)