Combined Community Codec Pack
The full title is the Combined Community Codec Pack. CCCP was created to replace the video playback packs provided by several anime fansub groups in favor of a single reliable pack capable of decoding practically any groups' files - without breaking compatibility for other formats. Since then it has grown into a pack that can decode more or less anything that you will run into during your travels on the internet.
This is free and can be downloaded here.