I don't know if you still want this answer but I've tried the few options that are still available there for my Clamshell G3, 366mhz, 576 RAM, 40 GB HD and, definitely, it works fine with ubuntu 10.10 although I haven't managed to make Airport work (I'm a real novice with Linux).

You can install ubuntu 10.10 using the ISO CD without problems, instructions are very clear to follow (use the alternative version ISO). Once installed, it is very custumizable (the true reason why I prefer it to Tiger), I can't configure Airport yet but with a wired connection works fine (with 9.10 Airport it used to work). Pidgin, Mozilla, OpenOffice, GIMP, and some other programs work slowly but steadily considering that it is a +10 year-old computer.

I've tried Debian Lenny which worked fine but with no sound, MintPPC 9 (a good option and it can still configure Airport), Xubuntu (to be honest I've found not much difference in performance compared to ubuntu) but works fine too (but again, no Airport), Kubuntu is heavy but it did run.

I chose ubuntu 10.10 and I'm happy with this distro because it lets me take advantage of my old but pretty laptop.