Free 3D Content Creation Suite: Blender
Blender is an open source 3D content creation suite.
It is an all-in-one package for 3D modeling, animation, rendering, post-production, and gaming.
User interface of Blender is formed of boxes, where all of them can be moved for the max. flexibility for not being blocked by the UI.
Advanced animations & the integrated game engine can be controlled with Python scripting which also makes developers a part of the game rather than only the designers.
A very detailed feature list can be found here.
The software has a large community & is very well-documented which makes it easier to improve your skills.
An open source game built with Blender:
Yo Frankie! is an open source game built with Blender & its integrated game engine.
The game is also like a source of information (as you can download the source files) for anyone planning to develop games with the application.
P.S. Anyone into 3D design is probably aware of Blender but this post is published considering it may be helpful to fresh 3D designers.
Compatibility: All Major OSs