Airfoil 3
One of my favorite Apple products ever has got to be the Aiport Express. I know it isn't sexy, but AirTunes (that's what Apple calls the technology that lets you stream your music from iTunes to a stereo connected to an Airport Express) is one of the best tech inventions ever. AirTunes, despite my unholy love for it, has some drawbacks: it only works in iTunes, and it only works for audio files.
Enter Airfoil, which just hit version 3. Airfoil allows you to stream audio from any program on your Mac to an Airport Express (or multiple Airport Expresses) and keep all those streams in sync. That's not all, version 3 lets you stream audio from one Mac to another Mac that is running Airfoil Speakers (a mini app that comes with Airfoil), one upping Apple's solution. Airfoil also includes a new video player that allows you to stream audio from most video formats to Airport Expresses (or other Macs) which will make watching some of those HD trailers a much better experience.
Airfoil 3 costs $25, but is a $10 upgrade for all Airfoil 2 users (and if you bought Airfoil after July 16th, 2007 you get a free upgrade).
