Heh, back in the quake1 days, one of my friends used mouse1 for forward and mouse2 for back.
Me? I use F to jump. Spacebar is too flimsy, and I have a hard time timing jumps right using 2 different hands for movement. Of course, it would be uncomfortable to use W for forward with my finger on F, so I use S for forward -- all in a row, A, S, D, F: Left, Forward, Right, Jump.