The Combined Community Codec Pack Project (CCCP) is a collection of useful codecs for playing just about any kind of video file you're likely to encounter online. The whole point of the CCCP is to provide a comprehensive set of codecs that will not only play common files but also uncommon ones.
It's recommended that you use the CCCP in conjunction with CCCP Insurgent, a separate piece of freeware that identifies any codecs currently installed on your computer so that you can disable them according to the instructional wiki on the Project's Web site. The CCCP is focused on decoding video files, not encoding them, so there are issues with some video production and editing software--for instance, it doesn't play well with some Nero components, and it asks to disable them when you install the pack. However, it's worth the work to see the unusual anime shorts and demos these fan groups produce. The download also includes the excellent 321 Media Player Classic, which works when Windows Media Player won't.
The CCCP Project maintains extensive instructions and documentation on installing, using, and troubleshooting the pack, including known conflicts and issues and how to resolve them. The codecs are ready for Windows 7.