Jaadu is my desktop Mac's missing multitouch touchpad.
You might not like this as much as I do, but it's really stinkin' cool. With this software, you can use your touch screen as a touch pad or remote for your Mac. You can control movies or iTunes with a touch. I love this one, but YMMV.
If this app sounds cool, but you don't think you'll use the full functionality, check out Jaadu Click. It does a lot of the remote functionality, but is significantly cheaper. It's also a great way to test the app before dropping a handful of cash for the full meal deal.
