Have you heard about Songbird yet? Think Firefox meets iTunes. It's your one-stop shop for everything multi-media. Open it up and play your iTunes library. Or browse to your favorite website, and Songbird will serve up in a sub-window any and all media found. Find a tune or a movie you like? Add it to your Songbird library and it's yours to queue whenever you wish. I'd try to sum up all what Songbird can do, but this video demonstration is about 1000 times easier to understand.
My vote for coolest app of 2007.