Altitude, which has been briefly mentioned on DC in the
RoboKill 2 thread, is a simplistic and fun 2D airplane combat game.
Best described as a cross between Teeworlds, Team Fortress 2, and an RPG, Altitude is a shockingly addictive online multiplayer game from indie studio Nimbly Games. Grab a plane and take to the skies as you compete (and co-operate) with other players and earn experience points bombing the enemy base and taking down foes with a variety of power-ups. As you gain levels you unlock new planes along with equippable "perks" that enhance and alter each vehicle. The instant action grabs you from the start while the nuanced gameplay and healthy serving of upgrades keep you in for the long haul.
Best of all, you can try the full version for free this weekend (right now!) on
Steam (Windows or Mac), and if you like it, you can buy it for $5 USD until Monday. It is also available for Linux, but you'd either need to buy it on Steam and wait for the Linux version of Steam, or buy the Linux version direct from
Nimbly Games.
FULL DISCLOSURE: I am in no way affiliated with Nimbly Games, however, they do have a referral program that will get you in-game points with which to "buy" skins for your airplanes. The above link to Nimbly games will give me some of those points just by you clicking it. As far as I know, there is no way to get the referral bonus for referring you to the Steam version, so if you're feeling generous, click the link anyway. If you don't want me to benefit at all for sharing this cool game with you, then don't click that link, and you may still visit the site at:
http://www.altitudegame.com/ 
