VLC media player

VLC player is great because it is extremely versatile. While it lacks the polish you might find in another media player (and thankfully, also the bloat) you can get it to play *nearly* any type of multimedia including DVDs (VOB) and Flash Video (FLV), and it can even transcode between a huge variety of formats. It is also good at playing incomplete or damaged files, including partial downloads and damaged DVDs which can be handy.
It runs on a nice variety of platforms as well.

It's not my primary player, but it's a reliable workhorse and a must to keep around.

If you use a VLC Video Player, Don't need download codecs anymore. This player include the lastest version for this codecs and play any format.

Good

This is a nice A/V-player. I use it exclusively for video files, and it works just fine.
It has, nonetheless, some serious limitations. For example: it does not support Real* stuff which makes those kind of streams problematic, and it has a really cumbersome interface when in full-screen mode.