Old iBooks are still around and most you can find for very cheap if not free as it was in my case. Ubuntu Linux runs quite well, but I must admit that my love of lengthy compiling and customization leave Gentoo as my primary mainstay.

make.conf for iBook PowerPC 750/745/755 (G3):

CHOST="powerpc-unknown-linux-gnu"
CFLAGS="-mcpu=750 -Os -pipe"
CXXFLAGS="${CFLAGS}"

sysctl.conf modification:

vm.swappiness = 20

For more information, check out these two links:
http://forums.gentoo.org/viewtopic.php?t=175419
http://en.gentoo-wiki.com/wiki/Safe_Cflags/PowerPC