Windows: SimpleSndVol adds balance controls to the volume control popup you get when you click on the sound icon in the Windows systray. The utility also allows you to bind volume and balance controls to keyboard shortcuts for quick access.
In Windows Vista and 7, balance isn’t available from the systray. You can always open the mixer or sound options to tweak the balance, but if it’s there in the taskbar, there’s no need to. In additional to balance control and keyboard shortcuts, the app also saves volume settings for individual applications. SimpleSndVol is free, and works in Windows Vista and Windows 7.