Command to convert movie to animated GIF, using FFMPEG

This command makes an output GIF that is 1000 px wide, 6 seconds long, with 10 frames per second:

ffmpeg -i source.wmv -vf scale=1000:-1 -t 6 -r 10 output.gif

Here are some more FFMPEG commands: http://www.labnol.org/internet/useful-ffmpeg-commands/28490/