Sound Control - App-specific volume control
Muzzy - it’s a tiny and gorgeous iTunes companion that lives on your menu bar. For Mac OSX, its free!
EarTrumpet - Volume Control for Windows
Skip Tunes - Control iTunes, Spotify, or Rdio from your Mac's menubar
Volume2 - Volume2 is an advanced Windows audio mixer, a complete replacement for the standard Volume Control.
Plaaying - Share, control, and keep on Plaaying music on your Mac.