Best Filesystem to Use When Moving HDs Between Unix Boxen?

gseidman asks "I've bought a large SCSI hard drive and I am planning on installing it on a Sun running Solaris at school for a while. Later, I expect to be using the same disk on a Mac (clone) running Linux (LinuxPPC, probably). If possible, I would like to format the disk so that it will work on the Sun and not require a reformat to work on the (Mac) Linux box, i.e. I don't want to have to do a backup and restore when I move it. Is this possible? What filesystem should I use?" Heck, why limit this to just Solaris and MkLinux? What is the best filesystem to use if you plan on sharing an HD between a varietyy of systems and NFS isn't an option?