Windows only: Freeware app CodecInstaller searches your computer for commonly used codecs and displays which codecs you have installed and which codecs you need.
CodecInstaller can even download and install missing codecs.
Additionally, you can point CodecInstaller at a specific file and CodecInstaller will tell you exactly which codecs you need to play the file.
CodecInstaller requires the .NET 2.0 runtime and is a free download for Windows only.

CodecInstaller [JockerSoft via Freeware Genius]