Livescribe Smartpen and was very impressed. If you're not familiar with it, Livescribe's device lets you write like a normal pen but it also captures everything digitally, including the audio around you. So it's perfect for taking notes in a meeting or in a classroom. One cool feature is that when you touch the pen to a note you wrote during a recorded meeting the audio playback jumps right to the point where you took the note.
Imagine this solution applied to the iPad. The "smart" components in the pen would move into an iPad app and the pen would simply become a stylus. The iPad even has a microphone built in to capture the audio from a meeting/class.
Oops. Anyone who owns an iPhone knows that the touchscreen doesn't respond to inanimate objects, so a stylus won't work on the iPad's similar display.
Bummer. I wonder if Apple would ever consider changing their display technology to support a stylus. A Livescribe-like application for the iPad would make the device even more powerful and irresistible.