Mac: Porthole streams audio from your Mac to your AirPlay device — or, more specifically for now, your AirPort (though Apple TV support is promised in the future).
With AirPlay, Apple has turned streaming audio from one device to another into child's play, which is great for folks with devices capable of acting as AirPlay destinations (which include the AirPort Express and Apple TV). Unfortunately, Mac OS X currently only allows AirPlay streaming between iTunes and AirPlay speakers (unless you feel like some more serious tweaking). Porthole adds an item to your menu bar that transfers your system audio to an AirPort Express with one click. It's a piece of cake.
The drawbacks: It costs $US10, which is a little pricey, but considerably less expensive than the go-to alternative Airfoil, which costs $US25. Drawback #2: It doesn't yet support streaming to the Apple TV. Fortunately, the developers claim that they're "feverishly working on support for the Apple TV", so hopefully that'll arrive soon.
Porthole has a free nagware version available if you want to try it out before you pony up.