Epic Games has found a wide audience, from Silicon Knights to Square Enix, with its Unreal Engine 3; meanwhile Crytek is looking to find success by licensing out CryEngine 2. Both companies also release games highlighting the engines (Unreal Tournament and Crysis, respectively).
Next Generation has an article where both Epic Games' Mark Rein and Crytek's Harald Seeley talk about the competitive field of game engines. According to the opening remarks, the article sees Crytek as one of the more promising competitors to the Gears of War developer.
It's an interesting read, but we shouldn't forget one engine that just recently made its first unveiling: id Tech 5. Creator John Carmack promised more information at Min-E3, and we think there's a good chance the House of Doom will add some major heat to all game engines.