In the little spare time I have, I hack a lot on Linux, mainly LinuxPPC. My hardware includes a PowerMac 7600, upgraded to a G3/300, an old Compaq 486DX/33, which serves as my print- and nameserver, an Apple Performa 475 running a Linux/M68K Debian 2.2, and a homebuilt PentiumIII/666.
Here's a bit about my hardware setup on the 7600.
I'm mostly concentrating on Linux for PowerPC, hacking away at the 7600's hardware.
A driver for PlanB, the video-in hardware in those machines.
Better support for the control display adapter. Some performance enhancements, and a way to sepcify the amount of VRAM present.
Updated: Hacking PCI fixup code for PowerMacs, making sure all PCI buses are correctly scanned, all devices enabled, and IO addresses correctly translated. This last point is still under heavy contention among the community...
Follow the link above for my current development patches against 2.2 and 2.4 kernels.
If you have a PowerMac upgraded with a G3 processor card, i.e. the box did not originally have a G3 processor, you might want to check out this page for a list of my problems and solutions.