Under windows: mencoder which I prefer over ffmpeg that some users have mentioned. It uses the same libraries and has -at least for me- a simpler command line

Under windows: mencoder which I prefer over ffmpeg that some users have mentioned. It uses the same libraries and has -at least for me- a simpler command line interface (mencoder file.in -o file.out -ovc select_video_codec -ovc select_audio_codec -of select_output_format -- try mencoder a -o o -ovc help -oac help -of help to see all options). Also its sister program mplayer is great for playing everything under windows - I also prefer it over VLC that has been mentioned here. It has a much simpler interface (only uses the keyboard - left right up down page up page down f o + - etc) than VLC and can be run through the command line (run mplayer test.avi and the video will immediatelly start). Finally, it can easily be integrated with windows explorer: Just add a "Send To..." mplayer shortcut and you'll be ready to play everything under windows! I'm using these tools for around 7-8 years for all my audio/video (transcoding and playing) needs, they munched everything I had thrown at them!