K-Lite Codec Pack - K-Lite Codec Pack is a collection of codecs, DirectShow filters and tools.
VLC Media Player - VLC is a free and open source cross-platform multimedia player and framework.
CCCP: Combined Community Codec Pack - The CCCP: Combined Community Codec Pack is a codec pack specifically built for playing videos (including mkv).
foobar2000 - An advanced freeware audio player for the Windows platform.
AC3Filter - AC3Filter is a high quality free audio decoder and processor filter.
Winamp - Winamp is a media player that allows users to play multiple-file formats, arrange media in a varied file-management system, and play unique media with AAC encoding.