TouchMouse is a free application for the iPhone/iPod touch that, when paired with its accompanying control software on your Mac or Windows computer, turns your touchscreen into a mouse and keyboard.
Once you download the software to your iPhone or iPod touch and pair it with the server software—Logitech has versions of the TouchMouse Server software available for Windows XP, Vista, and 7 as well as Mac OS X—you'll be able to use your device as a mouse for controlling your computer.
via TouchMouse Controls Your Computer’s Mouse and Keyboard via iPhone or iPod touch – Remote Control – Lifehacker.
I’m using it right now to write this entry. Did I mention it includes text input too? It does take some getting used to, but I can see that it would make great addition to a teknoids toolkit. Imagine having the server on all of those classroom PCs and an iTouch in the hands of every faculty or presenter.