Blender is a cross-platform open-source all-in 3D application. The realtime engine has a visual interface called Logic Bricks. But it's very limited. You quickly need Python to do anything slightly sophisticated in it.

Nevertheless we made Paradash with it, Guernica and the original version of The Kiss.

http://blender3D.org

http://kiss.entropy8zuper.org/the/incorporator

http://entropy8zuper.org/guernica

http://www.entropy8zuper.org/html/drafts/p-a-r-a-d-a-s-h