you can get good codecs by installing libavcodec (before you install mplayer). It supports most types of video (decoding only) and the performance is very good. To checkout libavcodec, type
server:email@example.com:/cvsroot/ffmpeg co login
(press enter when asked for a password)
cvs -z9 -d
server:firstname.lastname@example.org:/cvsroot/ffmpeg co ffmpeg
I'm not sure how binary distributions of mplayer deal with codecs, but if you're installing from scource, copy the 'libavcodec' directory from the ffmpeg source to the 'main' directory of the mplayer source. Compile should detect it there.