GameStudio A7 is mature, stable and complete game development solution, powered by -C like- Atari Lite-C language. Suite includes not-so-fancy model editor for creating and importing models, a world editor creating levels, and a full featured Script Editor with highlighting for Lite-C and HLSL, and command help. Features the Acknex 7 engine, an engine using Directx 9 for graphics with full shader support, ODE physics engine, particle emitters, complete multiplayer capabilities and plugin support. Comes in variety of licensing options, all royalty-free including the free version.
Check out this cool mode I am adding in the Silas singleplayer.