Paid-for version: Windows Media Player
Open-source alternative: VLC media player
VLC is an exceptionally functional media player that not only handles almost any media format you throw at it, but will stream stuff from the Web and play your DVDs. It's also a great tool for anyone who downloads large video files, since VLC can play incomplete or damaged media.
The Good: For the home and for the office, VLC will work for you. If you use Windows, Mac OS X, Linux or BeOS, VLC will work for you. Almost regardless of what media formats you use, VLC will work for you. If you're an absolute newbie to computers, VLC will work for you. Do you see where we're going with this?
The Bad: VLC won't let you sync your media library with your new MP3 player. True, Windows Media Player doesn't work with an iPod anyway, and the vast majority of other models that claim to need WMP will actually let you drag and drop your content into their memory through Windows Explorer anyway. There are no radio services directly available through VLC, so if the integration of these services is important to you, you might still want to have WMP sticking around in the background.
Conclusion: For almost all uses, VLC will suit the beginner and the amateur. The experts will already be using it. There are loads of skins available to make it look much prettier than it is by default, and its undeniably superb functionality makes it a crucial download for any computer user.
Download it here: videolan.org/vlc