Over on ATMac blog, Blake Watson has a great review of the HippoRemote app for iPhone/iPod Touch that turns your device into a multi-touch touchpad, as well as many other things…
HippoRemote will connect to your computer and hand the reins over to you. Your iPhone is now a fully-featured multi-touch trackpad. Tap the keyboard button at the top of the screen and HippoRemote will provide you with a standard iPhone keyboard… HippoRemote turns your iPhone or iPod Touch into an advanced media browser that interacts with some of the coolest media programs on your Mac….
life tools 
Geez, they’ve packed a lot into that little app! Looks amazing.
Though it is much more simplistic I saw NumPad on Adam Curry’s ‘Big App Show’ yesterday:
http://itunes.apple.com/au/app/numpad-wireless-numeric-keypad/id297623436?mt=8
It’s designed for those who have one of those little bluetooth keyboards without the numpad.