So one of the obstacles to using any Linux on Mac hardware is how to transfer files from ext to HFS. The first two pieces of advice I'd give is use ext2 (not ext3 or ext4) whenever possible, because while you gain a journaled file system (ext3) and extra speed (ext4), you lose reliability and compatibility. If you can help it, set things up so when you boot Mac OS you only need read access, not read/write.
For Mac OS 9: (MountX) (if anyone has a newer version, please share. This is an alpha version, it works, but as far as I know this software was developed until version 1.0d11, but those versions are mentioned online but no longer available for download).http://dl.dropbox.com/u/3809776/MountX_1.0a1.sit.hqx
For Mac OS X 10.4 Tiger: (MacFuse+fuse-ext2)http://dl.dropbox.com/u/3809776/MacFUSE-2.0.3%2C2.dmghttp://dl.dropbox.com/u/3809776/fuse-ext2-0.0.7.dmg
For Mac OS X 10.5 Leopard: (OSXFuse+fuse-ext2)http://dl.dropbox.com/u/3809776/OSXFUSE-2.3.9.dmghttp://dl.dropbox.com/u/3809776/fuse-ext2-0.0.7.dmg