Mousai was originally built to emulate the special function keys available on Apple Keyboards, for people (like me) with keyboards that does not have any special keys included. In addition to classic media playback and volume controls, you can also add quick access buttons for Launchpad, Mission Control, Dashboard and Eject.
Because Mousai emulates the real playback keys, you can control playback in not just iTunes, but all media players that support Apple Keyboards.
If you are using one of the supported media players, Mousai will also act as a mini player showing you notifications when track changes. You can also right click the menubar item to see the album cover of the current track.
Mousai currently adds these bonus features for iTunes, Rdio, Spotify and Vox.