EarTrumpet - Volume Control for Windows
Virtual Audio Cable - Ever wanted to record your speaker output? (loopback) This is for you.
Volume2 - Volume2 is an advanced Windows audio mixer, a complete replacement for the standard Volume Control.
Soundflower - Soundflower is a Mac OS X system extension that allows applications to pass audio to other...
KMix - KMix is an application to allow you to change the volume of your sound card.
Loopback by RogueAmoeba - Get all the power of a high-end studio mixing board, right inside your Mac!