Godot is free(libre), open source, MIT license, no royalties, 2D, 3D game engine full-featured. You can make any kind of complex games.
GDevelop is software for creating all types of games 2D and 3D without using a programming language. The interface consists of several publishers that allow the creation of the game. The program is completely free, and games created with run under Windows , GNU / Linux or the Web (HTML5 games).
Online multiplayer comes built in. Easy to learn language, and free. Perfect for a budding developer wanting to create online multiplayer games. Also provides many in-depth features to support their community of experienced developers.
Panda3D is a free and open-source game engine developed by Disney and Carnegie Mellon University. It has been used in commercial MMORPG games such as Disney's Pirates of the Caribbean Online and Toontown Online, as well as various indie projects. It is available under the Modified BSD License.