I love when I get to suggest products that I use myself here on The Porn Dude, because I feel as if I'm giving you the whole truth and nothing but. So yeah - VLC Media Player, or as my girlfriend calls it, 'the traffic cone program'. Put simply, it's a nifty, small, powerful and resource efficient solution for anyone that needs to watch a multitude of file formats on their computer. Best of all? It's free, open source and available for a number of different platforms!
I've yet to find a file that VLC is unable to play for the last 5 or so years of using it as my go-to media management system. It's perfect for video playback and can even do music if that type of thing tickles your fancy. It also has a playlist function, so if you're thinking of staying in for a day and monging out to a series, you can load them all up and never leave the comfort of your sofa to select a new video - neat!