Touchpad Pro turns your iPhone / iPod Touch into a wireless MultiTouchpad and lets you control your PC's mouse with your iPhone. And it's fast!.
There are a bunch of random things I've coded up over the years. None of these are up on the site -- if there's interest, email me!:
MousePedal: Turns a mouse into a MIDI controller. Useful if you're using your laptop as your guitar amp, and don't have a midi pedal handy.
UltraPedal: Turns a Behringer FB1010 into a full fledged Guitar Rig controller. Control delay/distortion/chorus/wah/whammy/loops/whatever with this app. Special thanks to Ossandust for his custom EPROM.
Siproxy: Lets you use Microsoft Speech Server with Asterisk.