You know the ones I'm talking about. The candy-colored ones that live forever, even as you're hoping they'll die so you can replace them with something that will run OS X? A few can be upgraded, but OS X is really made for something a bit, well, snappier. Instead of wishing for them to die, though, why not just install Ubuntu on them?
Ubuntu 8.04.1 is the latest and greatest from Canonical and isn't officially supported on older PowerPC hardware (Canonical has moved to AMD and Intel architectures exclusively). However, a community-supported version is available. The torrent for the text-based installer can be downloaded here. Don't bother with the Live CD version; it exists, but the graphics hardware on most of these machines just isn't up to the task.
My test machine was a 400MHz G3 iMac with 512MB of RAM and a 10.3 GB hard drive. The good news is that it works. The bad news is that it's fairly slow (no kidding, right?). The install only took about 45 minutes (not terrible for this processor), booting into the OS took a couple of minutes, and the rest was fairly smooth. The real value here is that students get an up-to-date web browser capable of running the latest Flash and multimedia applications and have OpenOffice.org 3.0. In many cases, even running slower than lightning speed, these are all students need. OS 9.1 or 9.2 that's running on on most of these old iMacs simply can't provide these tools.
I'll report back on student impressions once I get a whole lab running. We have a woefully underfunded alternative program that needs a computer lab; looks like they'll be getting some candy-colored Ubuntu-running iMacs.