VLC media player
Simply the best free multi-format media player
VLC Player is the most popular open source media player for a reason - it's excellent! It isn't the most attractive program though, and the online community has been hard at work creating lots of skins that improve, or at least change its appearance.
VLC Skin Editor lets you join in, but be warned, it's not an easy process. As well as being creative, you'll need patience! To get going you need to install both the editor and also the Java Runtime Environment. It's far from intuitive so check out the 'extras' above for an online tutorial - it's very necessary!
Before you create your skin you have to make PNG images for it with an external image editor (eg: GIMP), as VLC Skin Editor only allows you to add the various player attributes to the skins, not create the graphics. The step by step tutorial is pretty clear, but to get to a working skin will take quite a lot of work.
It's good to see an attempt at opening up skin design to a broader audience, but this is still far from a user friendly experience.