.png to .mpg
ffmpeg is a video encoder. It uses lavc, same that mplayer does. You can use it to turn a bunch of pictures into a movie. First, have it save them all with a predictable filename (ie, numbered by integers). Then run ffmpeg and tell it where the inputs are and where to output.
ffmpeg -i 'pngs/%05d.png' -hq 1d_freeend.mpg
ffserver is theoretically a streaming video server, and appears to be pretty good. I could never get it to do anything useful... but vlc and other videolan things seem more promising.
.avi to .flv
ffmpeg -i input.avi -y -sameq -ar 44100 output.flv
.flv to .ogg
to take the audio from a flash video file and convert it to ogg vorbis,
ffmpeg -i in.flv -vn -acodec vorbis -aq 100 -ac 2 -y out.ogg
-vn no video
-aq audio quality (vorbis specific ~ values 1-100)
-ac audio channels