Godot is free(libre), open source, MIT license, no royalties, 2D, 3D game engine full-featured. You can make any kind of complex games.
GameMaker is a beginner friendly 2D game engine that has "visual scripts" so no program skills are required. Platforms: HTML5, Linux, Windows, OS X, Windows Phone, And...
Platforms: Linux, Mac OS X, Windows, Android, HTML5, Wii, GCW Zero
Platforms: Windows, OSX and Linux