Linux only: Free utility Ear Candy makes your sound system smarter. If you're listening to music and a Skype call comes in, or you load a YouTube video, Ear Candy gently lowers your music volume to let the other sounds through.
Ear Candy does this for a few different sound "events", including sound files, movie and VoIP calls. It's basically a volume manager for PulseAudio, the new (but not necessarily loved) audio system for many Linux distributions. Ear Candy has a few other tricks up its sleeve too, like automatically switching your mic input and audio output to USB headsets when they're plugged in (which is, unfortunately, not something most Linux desktops do perfectly well on their own). It's certainly not bug-free, but I found it pretty cool to watch my Big Star soundtrack automatically phase down when I clicked on an emailed YouTube link.
Ear Candy is a free download for Linux systems only. Ubuntu users can add a PPA for updated installations, while other Linux systems can install from source.
Ear Candy (a sound level manager) [Launchpad]