Span two monitors from two separate computers, and use only one keyboard and mouse.

I use OSX2X to span an iBook and a Win98SE PC, using the iBook's keyboard and trackpad (well, actually I use a USB mouse most of the time), as described in more detail here.

I just found the program that does the reverse. Control two computers from the keyboard and mouse of one Windows machine (95-XP), while spanning their monitors.

I have not tested WIN2VNC, but I use OSX2X daily. Constantly.