Windows only: Ever download a video file but can't for the life of you get the thing to play back for you? Freeware application CodecInstaller detects the audio and video codecs already installed on your system, analyzes video files to determine what codecs they need, and helps you download and install them if you don't already have them so those unplaying files start playing again. Not everyone needs this sort of granular codec support (especially if you're using a player like VLC), but if you have run into this sort of situation, CodecInstaller should come in very handy. CodecInstaller is freeware, Windows only (just avoid the toolbar installation when you install it).
CodecInstaller [Jockersoft via FreewareGenius]