DrDJ is a little applications for DJ Wannabe's. It can play MP3s, AACs (and any audio file CoreAudio can play) and allows you to fade Between songs. You're in controll of the transition, but DrDJ can do everything automagically too.
The on-the-fly playlist can contain an infinite (okay, so it's limited by the size of your computer's memory) number of songs, so you can fill it, let DrDJ handle everything and go enjoy your party! With the new support for your apple remote, you can even decide from a distance when to start the next song.
