A very popular media player, made by Nullsoft, for the Windows operating system. It is very fast, stable, and has a huge variety of options.
It can play a variety of formats, including:
MP1, MP2, MP3
... and many others
It's functionality can be extended by downloadable plug-ins. It also offers the ability to be "skinned", and has a feature for visualizations which can go along to any music playing.
Personally, it's the most stable program on my system. And one of the few I actually respect. :)
Winamp is available at www.winamp.com.