"Useless" is a bit harsh of a term for a 2 GHz dual processor G5 PowerMAC with 8 GB of memory. While they don't stack up to today's high-end PCs, those machines are still not too shabby, even 11 years after release, just a little neglected in the OS department. They can run Ubuntu and other flavors of Linux as well as FreeBSD, so they would be a good machines for someone who wants to explore those OSes.
--z