FFmpeg, by Fabrice Bellard, et al., is a quite incredible command-line-only multimedia converter, capable of converting a truly vast number of input and output media formats, and depending on which binary you use, supports either a HUGE number of control parameters, or a REALLY HUGE number of control parameters.
I find it most handy for converting YouTube FLV files, and the raw video from my camera. As well as converting DVD VOBs, of course.
You can basically convert anything to anything, whilst doing crazy stuff like mixing MP3 audio tracks with H264 video, and adding ID3 tags to AVI files. Big fun. Of course, it allows you do do things the correct way, too. No limits.
While converting, the console output from FFmpeg can be viewed live inside ffe, and when complete, the entire process log is available for viewing/searching.
Comparison of audio player software