Platinum Arts Sandbox Free 3D Game Maker is an open source game design program for kids and adults based on the Cube 2 Engine. Through the ingame and cooperative editing and focus on ease of use the program is easy enough for kids to use but powerful enough for full game projects. Sandbox includes Save The Princess Gameplay, an RPG Maker, a Sidescoller maker, a Movie Maker aka Machinima, Save The Banana Base Capture, and more! Sandbox is already being used in many schools and colleges throughout the world and included in game design contests.
This is my favorite engine.
Pros
Everything is easy to learn and understand, but it's still powerful.
The community is very helpful and it's actively developed.
You can create a variety of games with this.
Cross-platform!
Cons
Mapmodels decrease performance too much.
The Game Design Community/Social Site is not really used.
But overall a very good engine, so I give it 10/10 points.