The G5 was supported with Fedora until recently but alas support was dropped. Also the newer Power distros are the new ppc64le flavour, which is a slightly different ABI and little endian but the G5 is not alas dual endian so does not support it. But they are cheap and quiet and mine has been running reliably, although I might install FreeBSD on it next as that has ongoing support.
I tried all flavours of PPC linux and none of them worked particularly well. FreeBSD 10.x had the advantage of 64 bit support and the nvidia 6600LE works acceptably with the open source drivers.
I highly recommend FreeBSD 10.x PPC64 to anyone with a G5 PowerMac who wants to run a modern OS.
I still use one to test software because testing on "weird" hardware reveals more bugs.
But obviously not everyone does that, so some of the software in the package manager still has aforementioned bugs.
Since then it has all been about making it look good for enterprise goons. Consolidating and vertical scaling and all that.
And poor buggers that end up with high cpu steal and poor performance because a new box is just unimaginably expensive when you have filled the existing system.
There are really only two models worth having, at opposite ends of the spectrum: The PowerMac G5 (64-bit, based on POWER4) was the fastest available but they're enormous and power hungry. At the other end, the G4 mini is much smaller and more efficient but 32-bit and not as fast.