I use Media Player Classic (included into the K-Lite Mega Codec pack) to play video files like avi, .asf, .wma, .wmv, .mpeg, .mka, .mkv.

I also use Real Alternative and QuickTime Alternative from K-Lite Mega Codec pack to play Real Video/Real Audio and QuickTime with Media Player Classic.

And I use PowerDVD when playing DVD videos.

And I use Winamp 5 or Foobar to play audio files.

p.s. From the software above, only PowerDVD is commercial, but I got it with some device.