Apple has long had a policy of releasing a new version of OS X every 18 months. This is not bad in and of itself but it seems that every release introduces incompatibilities. Fortunately the versions are backwards compatible so that the software you bought for v10.3 will run on v10.4 and higher. But software that is built with a new version of the OS won't run on the older versions, even if they don't use features specific to the new version. Software developers compound this by developing with the new OS soon after its release making software that is incompatible with computers that have not been upgraded.
This was driven home recently when my Mac G4 running OS X 10.4 could not "un stuff" a "Stuff It" archive. I went to the "Stuff It" website to download a newer version of the extraction utility so I could get at the contents of an archive a colleague had sent me. It turned out that they didn't make a "Stuff It" extraction tool for my version OS X any more.
So it seems that you have to buy every new OS X release or, due to industry behavior, within at most another 18 months you will be incapable of buying or using any new software on your Mac. This makes me feel as if I'm "leasing" my Mac with a huge down payment. Pay the "Jobs Tax" or within, at most, 3 years you will be incapable of running any software you don't already own. Its rare, if indeed ever possible, to purchase old versions of software.
With this in mind and the release of OS X 10.6 that no longer supports the PowerPC (PPC) powered macs it appears that within a relatively short period of time those Macs will become orphans without any further viable software available for them...
But there is good news! Linux likes to run on all kinds of things. So instead of throwing out that old Mac give it a new lease on lif by installing Linux on it. At one time I had an old iMac G4 200MHz system. I was amazed how much more power it seemed to have as compared to a Intel PII-400. So the bottom line is that Linux runs real well on the PPC architecture.
The Linux software world is still a bit immature but there are many great programs out there to give your old hardware a new lease on life. You might even find it runs faster than when it did with Mac OS X on it. If you plan on replacing that PPC Mac with a newer Intel based one you could still put Linux on it and hand it down to someone else.
To get started in the Linux world you need to download what is known as a "distribution". Simply stated that is a copy of Linux and a suite of software for it. You can learn more about how Linux is distributed in this article. If you have the software necessary you can download and burn a CD / DVD image. If not you can typically purchase CD sets for just a few dollars. Most of the distribution sites will give you the option.
So to get started here are some distributions to look at:
Those are a few places to look to download Linux for the PPC. Be sure to look around for the user guides. You might even go back the site's home pages to see what is going on with that distribution. Happy hunting and I hope this helps to breath some new life into the abondoned Macs out there!