Coolest feature in Vista
Being able to mute and control the sound volume on individual programs... absolutely awesome. So many times, I've wanted this type of functionality.
It can be quite useful. For instance, over the weekend, I was playing a video game that didn't really need the sound, and wanted to catch up on some podcasts. Just mute the game and leave the rest of the PC normal.
Or say you are on Skype and don't want to go deaf when someone else IM's you. Then just mute Windows Messenger.
For that matter, could mute Messenger when doing a lot of other things. When listening to some music with the volume up, mute Messenger. When playing a video game at night with the headphones on, mute Messenger.
Now the one thing I would love to see: a Windows sidebar applet that lists the current applications and lets you quickly mute/unmute them. Right now, have to go to Control Panel, Hardware and Sound, and Adjust system volume. Not hard, but think a sidebar applet would serve nicely.