iPhone/iPod touch only: Freeware application Touchpad Pro turns your iPhone or iPod touch into a multi-touch mouse for any operating system. You'll need a jailbroken iPhone or iPod touch, then just install Touchpad Pro from Installer.app. To get it working with your computer, you'll need to install VNC (it's already running on your Mac) and follow the instructions on the site. Once it's running, you can use your iPhone's screen like a MacBook Air trackpad, which means you get the regular mouse actions in addition to two-finger scroll, double-finger-tap right-click, and a ton more (check out the video for the full demonstration). Touchpad Pro is freeware, iPhone/iPod touch only.
Introducing Touchpad Pro