The Drag[en]gine is a highly modular, open source (C++) game engine that has been under active development for several years. The Drag[en]gine’s modular approach is built around the GLEM concept breaking your game project into the Game Script, Launcher, Engine and Modules layers. The Game Script is implemented by default in Dragonscript, another open source project available here. Drag[en]gine is open source under the LGPL license on GitHub.
If you want to get started with Drag[en]gine you can download binaries for Linux and Windows available here, it’s most likely the IGDE file you want to start with. There are a number of samples to get you started available here. You can learn more about Drag[en]gine in the video below.