Cecconoid, a fantastic twin-stick shooter with small cramped environments where the screen flicks between each section actually released the source code and now you can play free. The source code snapshot on GitHub is a snapshot of the DRM free version of Cecconoid, so it's unsupported but gives you another great free game to play.
"The starship Equinox is under attack from Stormlord and his robotic minions, the Exolons! You're the crew's only hope. Take your Samurai-1 fighter, find Captain Solomon's Key, clear the decks of evil robots, and save the Equinox from certain destruction! Cecconoid is an 8-bit inspired, flick-screen, twin-stick-shooter, set in an alternate dimension where the pixels are still chunky, and the bad guys are black and white."
Note: the source code available on GitHub is technically Source Available and not "Open Source" since it's using a Creative Commons license rather than an OSI approved license. That said, the license is the very permissive CC BY 4.0 license, so you can even re-use the whole thing for commercial purposes if you wanted to.
Main game features:
- 50+ Rooms
- 6 different power-ups
- High Score Table
- Full, bonus arcade-game, Eugatron