Converting an Mpeg video into an animated gif:

avconv -i input.mpeg -s qvga -vf format=rgb8,format=rgb24 -pix_fmt rgb24 -r 10 output.gif

Update: The output was initially quite large, but opening and saving it with the GIMP brought it from 35MB down to 7MB.

Example output:

(It's the Bristlebot video.)

Converting a bunch of images into a video:

avconv -r 50 -qscale 2 -i images/%05d.png out.mp4

(This one was useful for the Glibbersim animations.)