VideoLAN's VLC is one of the most popular media players around, tackling most every media format under the sun with aplomb. The app has been in development since 1996 and recently hit version 2.0. It also just hit a rather important milestone: over one billion downloads.
VideoLAN didn't start tracking downloads until February 2005 and isn't making any guarantees on accuracy. The numbers also only account for downloads on Windows or OS X and the source code; Linux downloads weren't included, as most of those are made by the individual Linux distros and not necessarily by the users. It's an impressive number — if you've never checked out VLC (or want to help push the app to its next billion downloads) you can grab it from VideoLAN.