VLC is probably the most popular open source media player out there. VLC plays basically any type of media file you can imagine. It has a very trim lined interface. The program is very light on system resources as well. Not only is it an incredible media player, it is also a server and a client to stream media across your network or the Internet. Various tweaks in video and audio are limitless. VLC is completely open source and available on Windows, OS X, and pretty much every flavor of Linux and Unix.
This is the basic interface of VLC there are also multiple skins available.
VLC is capable of playing multiple video files including MPEG, DVD, VCD, Real, WMV, and a bunch more!
There are many cool visualizations you can use while playing audio.
VLC is definitely a top notch media player and can also be used for streaming video across a network and the Internet. If you are a coder you can download the code here. I highly recommend this player!