Logitech's Touch Mouse app [iTunes link] turns an iPhone (or iPod Touch) into a wireless touchpad and keyboard, great for when you want to control a PC from a distance. Take a look:
Touch Mouse relies on Wi-Fi to communicate with your Windows or Mac system. Consequently, you'll need to download and install Logitech's Touch Mouse Server utility (also free).
Of course, that also necessitates a Wi-Fi network, which may be a limiting factor depending on where you're giving your presentation.
As you can see in the screenshot, Touch Mouse provides left, center, and right "mouse" buttons. A tap of the keyboard icon brings up onscreen keys you can use for data entry.
The app also leverages the iPhone's multitouch screen: You can scroll by dragging with two fingers and drag and drop by holding the left mouse button and dragging one finger. Neat stuff.
In my tests, installing and using Touch Mouse was a breeze. This is one seriously handy app for remotely controlling a nearby PC, and unlike most other "air mouse" choices in the App Store, it's free.