Besides the already named ones, here a few:
- endless sky: Inspired by a game called escape velocity. You fly around in a spaceship and do space adventures.
- naev: Similar to above, but quite different.
- ddnet: Multiplayer coop 2D platforming. (Has enough active players.)
- warzone 2100: Real time strategy with tanks. Has a campaign.
- battle for wesnoth: Medieval turn based strategy game.
- mari0: 2D mario with protal gun.
- mindustry: Build factories and defend against enemies.
The first step towards becoming cybermen.