TidBITS Staff:

Can’t remember which key combination creates an e with an accent agu (é)? Press and hold a key to bring up accented alternatives, a feature introduced in iOS. You can click the accent you want, or, since your fingers are already on the keyboard, press the number that appears below the character you want.

I actually didn’t know this one and a few others on this list. One thing I do know: option is an incredibly powerful key in Lion.