foobar2000 (not capitalized) is a free audio player for Windows. Although it has a utilitarian, graphics-light interface, it boasts many features for metadata support and improving audio quality (so far as that is possible), as well as a number of official and third-party plugins for a variety of purposes.
http://www.foobar2000.org/ - The official webpage contains little general information, but it does have a forum where the development of foobar2000 is discussed.
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