Jason Snell, editor of MacWorld, has a column up recounting the good times he had playing network games of Marathon while working at MacUser, and insists that Marathon is really at the core of what Halo is:

In any event, don't let anyone tell you otherwise -- Halo really is Marathon 4, right down to the weapons choices and the gameplay. And I love it... especially on the network, because then I'm playing with and against real-live human beings with real skills. Or imagined skills, but that's fun too.

Thanks Ghôlsbane and Louis Wu at HBO.