Despite what is displayed on this entry, 0 A.D. is not a game that got released in 2010.
This game has been in development since 2000, at the time the plan was to develop it in the Age of Empires 2 engine, as a total conversion mod. The developers quickly felt limited by this engine, and this became a stand-alone game project using an in-house engine.
In 2009 the source code of the engine and game (including the art, not only the engine code) got released under an open-source licence, and the development had been ongoing ever since, with the most recent release (at the time of writing this) being the Alpha 27 published on 2025-01-30. If you don’t know what open-source is about, you can see that as the most long-lived "early access" game distributed for free, developed by a team of volunteers and without looking for any kind of profit.
Despite the game not being complete yet — it lacks a single player campaign with a story — it is already on the level of many commercial games with regards to single player skirmishes and multiplayer.