The list does not mention some of the now more popular players and decoders. These are either very stable, of high quality, or provide extended functionality.
Media Player Classic by Gabest
A great lightweight player that can handle many formats and host Quicktime + Realvideo if the codecs are installed. Not necessarily the most stable player, but it can play .ogm files, use directX, Overlays, dvd's and does **subtitling**!!
VideoLan Client AKA VLC
A good solid player (not perfect) that has some serious abilities as a streaming server. DVD playback functions seem more stable than Media Player Classic, and video quality is better when using a video out to a TV or secondary LCD monitor/TV.
Filters & Helpers
While not a video codec, it is the best subtitle manager I have found. It produces nice clean and sharp looking subtitles around.
Mentioned by others it is a great video and now audio (yeah, you heard me audio!) filter and post processor. Use as a replacement for DivX or XVid codec installs. It does not enable encoding to any format, but it does decode mpeg4 format videos (divx, xvid etc..). It can also ad raw formated post processing to Mediaplayer (built in windows version) to enhance the quality of the playback (color balancing, deinterlacing etc..). It is not quite complete but the basic functions are stable and has surprisingly good results. Addtionally the new user interface is very nice and it can play nice with vobsub.
I use the audio filter version in addition to the video filter version, but I have not played with too many of the options yet. I used the audio codec version to successfully boost the voice track of a dvd that was oddly encoded. Not quite as mature as the video filter version, nor has as nice an interface, and it seems to chew up more of my processor time, buuuut it does add nice functional options.