I first started working with CrystalSpace in 2003. Back then compatibility would change from version to version, building CS was a regular task and CEL was still an up-and-coming project.
Then this January the team announced the first stable release of CrystalSpace; one feature-complete and ready to be production ready. It's amazing how mature the project has become since then... documentation has flourished, CEL has become a fantastic development framework, titles can be developed with little to no coding and Blender integration is now ready for prime time and sponsored by the Blender project itself.
Not only that, CrystalSpace has finally hit the prime time: binary packages are now widely available on most Linux distros. Debian has been carrying packages for a while, but now native SuSE RPM's are available via PackMan.
No, you shut up.
Now people can develop entire titles while only focusing on content creation and scripting, allowing people to focus on what makes a game a game. Not only that, there are copious templates and examples for how to build projects. And you no longer have to build packages every night. And it's open source and readily available. It blows my mind.