Have you heard of 0 A.D.? It could give the Age of Empires series a run for its money. The attention to minor detail (like the details in the artwork on the buildings of various civilisations), the superb music, the battle formations etc would make you think it was a commercial project. And the fact that they did 3D better than Microsoft could (AoE 3 was a disaster) makes it even more impressive.
Another of my favourite OSS games is SuperTuxCart, it could give Mario Kart a run for its money too. Vey fun, with a very active community around it too.
