The FFmpeg project is an open source system being produced to play and record various video formats. Among other game related video formats such as those used by Westwood games, the program will also play back movie files as used by WC3, and possibly also WC4. It is a function performed very well already by HCl's movie player, but it's nice to see this kind of support from outside the WC community. If you'd like to try FFmpeg, the program will run on both Windows and Linux, you can download the source code from here. Unfortunately you'll need to compile it yourself.