LineIn

When I am at work, I usually wear headphones connected to my laptop, so I can listen to music or podcasts while also being able to hear instant messenger notifications.  But, sometimes, I want to listen to music on my portable music player.

Previously, this would mean that I wouldn't be able to hear instant message notifications, since the headphones would be connected to the player and not my computer.

LineIn is a free utility that lets me listen to my portable player, and computer audio.  What it does is let you configure that the audio on any input gets mixed an audio output

[via TUAW]