Thanks to Vidgamer for pointing this out.
Kai is free software that lets you play ‘system link’ Xbox, Gamecube, PS2 or PSP games over the internet. It runs on Windows. Mac OS X users will also need Amaryllis (and an Airport Extreme for PSP gaming). Amaryllis is also free but they do request a small donation.
I haven’t tested this yet, but plan to do so this weekend. Hopefully my Airport Express will do the job (I assume they say “Airport Extreme” meaning 802.11g, which the Express is.) But we’ll see.