Web Wars is an open source arena shooter similar to Geometry Wars, Robotron, or Death vs Monstars. It has 12 stages, 3 difficulty levels, 9 enemy types, and weapon upgrades.
This is the "desktop" release of WebWars. It should run on Linux (32/64), Windows (32/64), and Mac, though it has not been tested on Mac yet. Unzip the file and run WebWars.bat. Note that you need to have Java installed.