Convert avi to vcd

ffmpeg -i myvideo.avi -target vcd output.mpg

Rip a DVD file, then conver it to MPEG4 + Ogg Vorbis (for OGM)

ffmpeg -i movie.avi -f avi -vcodec mpeg4 -b 800 -acodec vorbis mpeg4.avi

Same, but with AC3 sound instead:

ffmpeg -i movie.avi -f avi -vcodec mpeg4 -b 800 -acodec ac3 mpeg4.avi

Be *sure* to see ffmpeg -formats for what you can encode to/from, there 
is a lot!  Much more than MEncoder

ffmpeg recommended addons (for DVD rip, see man ffmpeg)

ffmpeg -i movie.avi -f avi -vcodec mpeg4 -b <video bitrate(800)> -g 300 
-bf 2 -acodec mp3 -ab 128 mpeg4.avi

# Steve's tweaks:

-hq High quality
-b 1800 Don't forget -- default bitrate is 200!