Sure it’s old news, but I am pretty happy that Mac OS X 10.4 has a built-in VNC server.
You’ll still need a client, like Chicken of the VNC, but it couldn’t be much simpler to make work. Though, you could run a separate server app (even several instances of it) and work up a hack like this to allow you to have several people all logged in to the same machine (and getting different screens) simultaneously. And it’s also worth mentioning the VNC to Flash screencasting trick.
And…if what you really want to do is use and control a couple (or more) computers side by side, then what you might really want is some virtual KVM software (or, more properly, some keyboard and mouse-sharing software). Zach is super excited about Synergy for these things.