Codecs

AC3Filter is a high quality free audio decoder and processor filter. It allows media players to playback movies with AC3 and DTS audio tracks. Also it can process any audio track, much of processing options allows to adjust the sound in almost any way. It is possible to upmix any audio source to 6 channels.
GSpot - codec information. It establishes what video codecs (audio and video) are required to play an AVI file, determines whether these codecs are installed on your system, and isolates problems associated with these codecs.
K-Lite Codec Pack is a collection of codecs and related tools. Codecs are required to encode and/or decode (play) audio and video. The K-Lite Codec Pack is designed as a user-friendly solution for playing all your movie files. With the K-Lite Codec Pack you should be able to play 99% of all the movies that you download from the internet.
QT Lite is a lightweight version of QuickTime. It contains all the essential components and nothing more. This is an excellent replacement. It needs less disk space, has no background processes, has the ability to download HD trailers.
QuickTime Alternative will allow you to play QuickTime files (.mov, .qt and other extensions) without having to install QuickTime Player from Apple. It also supports QuickTime content that is embedded in webpages. The very user-friendly installation is fully customizable, which means that you can install only those components that you want. QuickTime Alternative Lite is QuickTime Alternative without Media Player Classic. Community