The players that we are using for audio/video playback:

VLC Player
Very light, support most of the audio/video codecs. Play both file and streaming. Recommended.
But it doesn’t support AMR audio codec.

QuickTime Player
We use it a lot at work. QuickTime PRO supports both decoding and encoding. But always crashes when playing rtsp stream.
It supports some special codecs but doesn’t support some common ones.

RealPlayer
Similar to QuickTime player, but doesn’t support multiple clients. It also used QuickTime as plug-in.
Compare to QuickTime, it also supports H263 P3 which QuickTime doesn’t support.
Exclusively for RealMedia.

Windows Media Player
Too heavy for the system, but bundled with Windows (damn it!). Still useful in some cases (like MMS or WMV/WMA).