With my girlfriend in the process of refurbishing her kitchen, I found myself in need of (and without) a carpenter's level yesterday afternoon. But then I remembered Bubble Level, an application for the iPhone and iPod touch.
A penny short of a dollar later, I had myself a working, surprisingly accurate level right in my iPod touch.
Bubble Level allows you to calibrate it (for example, with an actual bubble level) so its measurements are accurate enough to use for household chores and hobbies. (Professionals may still need the precision that a traditional liquid bubble level provides.)
Bonus: it tracks how level the device is in two dimensions: laying your iPod or iPhone on a table allows the bubble to travel toward the center circle on the interface.
Bubble Level is 99 cents, and available in the App Store now. You can also check out A Level, a similar app that's expected to support calibration in the next release (it's from Posimotion, winners of TUAW's Most Risque iPhone App Name award).