I've said before how much I love the iBook G4 we have at home. I can't quite remember the specs, although I imagine it hovers around 1 GHz in speed, has about 384 MB of RAM and the Combo drive. It's running OS X 10.3.9 right now, but I imagine it would go to 10.4 easily. If you can get your hands on one of these, or a Titanium Powerbook from the same era, I strongly recommend it -- you'll have an OS X-friendly laptop for years to come.
The fact that they still bring so much money on the used market is a testament to the value of Mac equipment in general. If you don't believe in the "This Old Mac" ethos, you can easily keep yourself in new hardware by selling off the old before it gets too, too old, and plowing the money back into the latest and greatest. Of course, since Macs that aren't the Mini seems to start at $1K, you'll have to keep plowing money in ... and then there's all the new software and peripherals you'll need. It can run into some money.
In fact, software alone is both a plus and minus for older computers. On the plus side, you get more use out of old applications that either you or the computer's previous owner purchased and installed. The presence of Microsoft Office 6.0 on the Powerbook 1400cs was a prime motivator in the project that led to this blog. Sure, you can't get a modern browser on the thing, but there's little you can't do as a writer with the 1995-era Microsoft Word. You might not be able to open MS Word docs created in the most modern versions, but your documents can be read by those newer versions, and you can always a) convert or read those docs on a newer computer or b) ask the person to "save as" in a version that you can read.
And for Windows PCs, I've been pretty happy using Open Office to crack open every MS Word doc I've come across. My spreadsheet skills are rudimentary, but OO's version of that application has come in handy as well, since there are lots of spreadsheet documents on the Web (like Nielsen Media Research's TV ratings, which I access at the Daily News).
I wish SoundEdit 16 was still on This Old Mac (Ilene used it in her record company days), but it's mysteriously disappeared. And I don't think this PC is the ideal photo-processing platform, but if I came across a copy of Photoshop, I might install it. I'd love to find Dreamweaver, the Web-publishing program, for Mac's System 7.6.1 -- that I could use.
Trolling around for this old software, either on eBay or on swap lists like LEM Swap, often leads to such old software at very attractive prices. A search yielded an old listing for SoundEdit 16 for $5 (the program is still available new for $15o or so) ... but the listing was months old, and the CD was sold. You just gotta keep looking. And if you have old, unused software that you think is obsolete, check the market for it on eBay -- you could make some idiot like me very happy while making yourself a few sheckels richer besides.