Altitude is a fast-paced, online multiplayer aerial combat game with 2D physics-based gameplay and a cartoony aesthetic. It features 5 unique planes, a perk system inspired by CoD 4, crazy powerups, a map editor, and an account system that helps you find friends for maximum enjoyment.

LINK to Altitude
Being in java this (should be) is playable for mac and linux too.

The Demo gives you the ability to experience the game before making the decision to buy a full version of the game. There are no restrictions on the servers that Demo users can choose to play on, which gives a wide variety of choices of the game style and the skill composition of the players in the game. The Demo also allows you to continue to level even when the content unlocked for those levels has its use restricted by the Demo. This allows you immediately gain access to the features you have unlocked, but cannot access in the Demo, if you choose to Buy the Game.