If you have the right video card, you can get Mountain Lion running with these instructions: http://www.jabbawok.net/?p=47
This worked for me with my 2006 Mac Pro 1,1 w/ an ATI HD5770 graphics card. I do have an issue where it often doesn't come back from sleep, so I never put it to sleep, but other than that it's been flawless so far. I haven't spent any time looking into the sleep issue, maybe it's a solvable problem.
By strictly hardware performance standards, the Mac Pro 1,1 and 2,1 did not deserve to lose support so soon, and are still more than adequate. Especially the 8-core versions, which have benchmarks higher than many 2012 Macs.
If you have a 4 core model: I researched putting in replacement CPUs a while back (but never went through with it). Replacing a CPU in a Mac Pro is not the easiest and is a bit of a pain from what I hear, but it is possible, and if you have 4 cores right now you can get a serious CPU performance boost with the right 8 core CPU upgrades (2 x 4-core CPUs, that is). There are many options that will will work. If you're interested, I can give you more details on which CPUs work.
I have the four core model; I'm not sure for my work flow that faster CPUs or more cores would really help much compared to the cost. My SSD upgrade has really given it new life in terms of responsiveness.
Thanks for the link to the workaround. Hopefully it isn't something that will get broken with updates.
Yeah, I'm a little concerned about updates breaking things. The good thing about this hack is that you don't have to modify your installed OS X system. If I understand correctly, the Chameleon bootloader emulates EFI64 and fakes OS X into thinking you booted an EFI64 system. So I think the likelihood of updates breaking things is much smaller than that of Hackintoshes, for example.
Nevertheless, I'm going to try to make sure I have a full backup before doing any major OS X updates. I also have both a Lion backup (as well as an old Snow Leopard backup) that I can boot up in case I need to.
This worked for me with my 2006 Mac Pro 1,1 w/ an ATI HD5770 graphics card. I do have an issue where it often doesn't come back from sleep, so I never put it to sleep, but other than that it's been flawless so far. I haven't spent any time looking into the sleep issue, maybe it's a solvable problem.
By strictly hardware performance standards, the Mac Pro 1,1 and 2,1 did not deserve to lose support so soon, and are still more than adequate. Especially the 8-core versions, which have benchmarks higher than many 2012 Macs.
If you have a 4 core model: I researched putting in replacement CPUs a while back (but never went through with it). Replacing a CPU in a Mac Pro is not the easiest and is a bit of a pain from what I hear, but it is possible, and if you have 4 cores right now you can get a serious CPU performance boost with the right 8 core CPU upgrades (2 x 4-core CPUs, that is). There are many options that will will work. If you're interested, I can give you more details on which CPUs work.