FFMPEG is an audio-video transcoding engine to trancode one video format into another and of course lots of more. FFMPEG has been very popular in the Flash community because it supports the FLV Spark H.263 (Flash 6/7).
You probably know the Riva FLV Encoder I build in the past which also used FFMPEG.
Check the FFMEG GUI here and look into the URL-Parameters which you change of course.
What you have to do is to enter the path of ffmpeg.exe and of your input- and output-video. Then change the parameters to your needs and copy the command with the button into your clipboard. Open the commandline (Start-Execute->cmd) and paste the command.
The current FFMEG GUI lacks of validating the input-data. Validating data is handled very nice in Flex but I like to integrate it in the next version. If anyone likes to support on this feel welcome.
The sourcecode of the FFMPEG GUI is available here. The HTML for the sourcecode is generated by Flex 2. What a nice feature!