VNC provides a terminal emulation of many operating systems (ie. windows, many UNIX environments, and I'm not sure if they're at the Apple OS's yet).
It's freeware. The inherent disadvantage is that it cannot control the machine until it's completely booted, ie. no direct KVM control, just remote control once the machine is booted and has started the VNC server.
Search for VNC on google.
If you don't need actual KVM control, ie. need to be able to remotely control every aspect to the machine, go with VNC.