I still keep a couple of old Mac computers running. A favorite is my old Pismo Powerbook, followed by the Clamshell iBook. The Clamshell runs about 10 hours on a charge, and I have four working batteries for the old Pismo, giving me about 2-5 hours each.
While it's not easy watching a long DVD on the Pismo due to battery swapping, the iBook is great for that, and the Pismo checks eMail and surfs Facebook and the like just fine. Sure, there's no Flash and some of the JavaScript intense pages are slow, but you'd be suprised how far a 450mhz G3 takes you on the modern web!
Well, using these computers you need to have a modern, and hopefully optimized web browser to keep up, and with the "ooh, SHINY" crowd out there it takes a littile work to uncover a browser to work on these "old" machines.
A good place to start is the Firefox Mac Community Builds They have G3 builds (a little behind), G4, and G5 builds. There's also TenFourFox, which has some of the graphics acceleration stripped out, but runs faster Java and is completely 10.4 compatible.