Multibox -> very easy to setup. However, after a bit of inactivity on the second PC, the whole thing will just stop working. And I have yet to get it to work again w/o having to restart both PC's.
I haven't had that happen to me. I leave my dual-boxing systems running most of the time with Multibox running on both, and have no problems when I go back to them. I'll wake both systems up with one big mouse sweep.