VLC Media Player
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Full user review
"Good--but not perfect--player for just about anything"
Aside from the obvious pros of free and open-source software, VLC Player is able to play just about any type of media you throw at it. There are also some additional features, like streaming web content, capturing video/screenshots, etc. The fact that it plays DVD video without having to purchase a codec is awesome.
The interface has devolved from a solid, compact application into a giant, bloated one. It is becoming too feature-rich for its own good and is beginning to look and feel more like Windows Media Player and RealPlayer, which is precisely what I wanted to avoid. Apart from this, it's not exactly stable. Though it works 95% of the time, it does hang during use (and even crash my system) on occasion.
VLC Player is great if you own a library of media of a variety of formats or even if you're a new user looking for an alternative to WMP. If you want something bare-bones and even more reliable, I recommend Media Player Classic.