I found this article recently on Digital Tutors. It has some good content in it about choosing your game engine. The article addresses the main free engines out there (all good quality), Unity, UDK and CryEngine. http://blog.digitaltutors.com/unity-udk-cryengine-game-engine-choose/ Source
bah, that doesn't touch on much at all.... There are A LOT of engines out there... some of which are free, some not so much... http://en.wikipedia.org/wiki/List_of_game_engines http://www.worldofleveldesign.com/categories/level_design_tutorials/recommended-game-engines.php http://www.moddb.com/engines/top and, that's just the tip of the iceberg.
That article was written prior to the Unity 5 preview, which adds a lot more features, and lifts up the graphics quality drastically :) Disclaimer: I work for Unity.