Logitech's free Touch Mouse app turns your iPhone or iPod touch into a keyboard and trackpad for your computer
Touch Mouse [iTunes link] is a free iPhone / iPod Touch application from Logitech that turns your iDevice into a wireless keyboard and trackpad for your Windows or Macintosh computer. It's not a remote control application, so you don't get a view of the computer's screen; you simply get an alternative way to control the mouse pointer and keyboard. Touch Mouse works by relying on a small server software component that you install on your desktop computer that listens for connections from the iPhone app.
This is very useful when you have a computer set up as a PVR. Most of the time, you can do what you need to do with a remote control, but occasionally you'll need to navigate the file system or close a pesky dialog box (no, I don't want to upgrade my antivirus, thanks), and for that Touch Mouse can be very useful.
My one small concern about Touch Mouse is that there is no authentication going on; it appears that any computer that is running the Touch Mouse server software on the same wireless network shows up in the Touch Mouse app. That won't be a problem for me since I'm using it at home on my home network, but it could be a problem in other environments.