EarTrumpet - Volume Control for Windows
Volume2 - Volume2 is an advanced Windows audio mixer, a complete replacement for the standard Volume Control.
Sound Control - App-specific volume control
Volwheel - VolWheel is a little application which lets you control the sound volume easily through a tray icon...
Background Music - Control the audio on your Mac apps from the menu bar
Volti - Volti is an GTK+ application for controlling audio volume from system tray/notification area.