iPhone/iPod Touch: TouchMouse is a free application for the iPhone/iPod Touch that, when paired with the appropriate control software on your Mac or Windows-based computer, allows you to use the touchscreen as a mouse and keyboard.
Once you download the software to your iPhone or iPod Touch and pair it with the appropriate 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. The interface allows you to click the mouse buttons, move the cursor via the touch screen, and pull up a small keyboard to enter text onto the computer.
While such an arrangement isn't a practical replacement for a full out wireless keyboard it is a great tool for presentations or for pairing with one of the awesome media centres we've highlighted. If you have another app for turning your iPod Touch or iPhone into a remote or clever things to do with them once you have them set up to be remotes, let's hear about it in the comments.