I've been a big fan of MaxiVista for a long time. I love multi-monitor productivity and now that I have a TabletPC, I consider MaxiVista to be a required tool.
Now Version 2 is out, and it includes a nubmer of new features, with the shiniest of them being it's a KVM out of the box! With a hot key you can switch from your virtual monitor to the underlying machine. Automatically moves the clipboard contents, too.
The other thing that's pretty cool (and a little creepy) is the support for Video Playback. I'm not sure how they are doing it, but I can drag Windows Media Player to the TabletPC (over WiFI) and they virtualize the hardware surface such that the video just keeps playing.
I haven't tried the other new feature, but I'm hoping to soon. You can support up to three additonal monitors. I may just take over my wife's machine when she's not around, to expand my working desktop to a full FOUR monitors.
Check out the demo and put that extra machine and monitor to some use.
Now playing: Miri Ben-Ari featuring Scarface & Anthony Hamilton - Sunshine to the Rain