MPC-HC is an open source media player for Windows.
The program opens just about any audio or video format, along with CDs, DVDs or DVDs, and standard playback controls ensure you'll feel at home right away.
MPC-HC is vastly configurable. The interface can be customised, keyboard shortcuts remapped, file associations set or cleared, playback defaults defined, and a host of small features enabled or disabled.
Video experts in particular will love the complete control they get over filters, shaders, renderers, and every other aspect of playback.
The end result is supremely flexible. MPC-HC doesn't just have an "always on top" setting to keep its window above everything else, for instance - you can enable this only when you're playing video, so the window is visible when you need it, but can be safely hidden when you don't.
There are some handy extras scattered around the menus, too. Creating a contact sheet of frames taken from a video normally required a separate application, but MPC-HC can do it in a click or two (File > Save Thumbnails).
But despite all the functionality, MPC-HC is very lightweight, runs happily on otherwise underpowered PCs, and has plenty of optimisations to improve performance even further, if you need it.
One of the best media players around, easy to use for novices, but with the power and flexibility that experts need.