Converting mp4 to gif is handy for posting short screencasts. You can convert to gif using ffmpeg and optimize with imagemagick. To install:
apt-get install ffmpeg imagemagick (linux)
brew install ffmpeg imagemagick (Mac)
Convert to gif:
ffmpeg -i my.mp4 -pix_fmt rgb24 -r 5 my.gif
Optimize with:
convert -dither none -layers Optimize my.gif my_optimized.gif
UPDATE: There's also licecap and subsequently optimize with:
cat source.gif | gifsicle --colors 256 --optimize=3 --delay=15 > target.gif
UPDATE: Also consider for .mov:
ffmpeg -i in.mov -pix_fmt rgb24 -r 10 -f gif - | gifsicle --optimize=3 --delay=3 > out.gif