Icons
I've always liked icons. I've been designing them for years. The challenge is similar to that of designing a logo or even writing a haiku: taking an often complicated or abstract concept and boiling it down to a succinct, intuitive picture.
My favorite icons don't stop at simply describing a function, though. They're quirky and whimsical, sort of irreverent, like the little paper-airplane icon for the "Send" button in Apple Mail, or Google's Translation icon, or - most famously - the ubiquitous dog-ear on document icons you see in nearly every modern desktop environment. The dog-ear doesn't need to be there. A rectangle with lines in it would convey the message just as well. But it wouldn't have the same element of humanity to it, which I think is crucial to good user interface design: user-friendliness means, well, friendliness. Friendliness means personality. Computers don't dog-ear files, people do, and that extra touch of people-ness makes the entire interface friendlier.
These are the kinds of icons I try to design myself.
