Keyclone - fairly easy to setup - check the software section of the forums
Hotkeynet - also fairly easy to setup
AutoHotKey - not hard to setup if you use a prepackaged WoW script from their forums - but be very very careful to avoid *ANY* delay functionality that people might have added into those scripts... even if it seems harmless. There's some info on this in our software forums
Octopus - very nice app, bit more technical setup but very tweakable if you're into that. Check the software forums here for info.

Those are the software options i've tried when running multiple clients on one machine, I am sure there are a few more options out there that haven't been listed.