Altitude is a dogfighting indie game by Nimbly games. It's been out since 2009 and has a bit of cult following for its many inventive game modes and across-the-board support for Linux, OS X, and Windows.
Altitude combines the look and feel of a 2D side-scroller with Call of Duty style perks that give you the ability to choose from many different planes and weapon combinations. There are also power ups and special weapons scattered throughout the playing field.
You can choose to play casually or go for a more tactical game style through seven different game modes.
One of the most popular game modes in Altitude is called Plane Ball, and this is basically what we in the US call soccer combined with aerial combat. Your job is to get the ball into your opponent's goal, and they will try to stop you by any means necessary.
Altitude is a great game to pick up, even years after its initial release. Because of its cartoony graphics, it's actually quite evergreen. You will have a hard time finding an active server during some hours of the day, so this is a game best played with friends on your schedule.