License: Open source
From MediaKeys website:
MediaKeys is a free, small, portable program for Windows, which runs from your system tray, that allows you to set specific keys (now fully customizable!) on your keyboard to control your media player globally. This works even if you haven't invested in an overpriced keyboard with built-in media keys but if you do already have one, MediaKeys offers a greater level of control and has fantastically useful extra functions. The controls available are Play/Pause, Skip to Next Track, Skip to Previous Track (the last two can be held down to rewind / fast-forward the current track).
It is written in AutoHotkey, source is avaliable here. MediaKeys is not available in PortableApps.com Format.
Download here single .exe file.
Other apps (also portable, but not in PA.c Format) by bambi4 can be found at bambi4.co.uk