A mashup of Wolfenstein 3D and Minecraft. You can hit Enter/PageUp/KeypadEnter at any time to switch between editor mode and action mode. It's sort of a Wolf3D construction kit.

(If the default level takes a while to load, that's normal.)

Update: v1.02

WHAT IF MINECRAFT HAD PHYSICS?
Place blocks using the left mouse button, remove blocks with the right mouse button or with the control button.

Native version for Windows and Mac:
http://wolfos.org/resources/Epic/native.zip (16MB)
Recommended so you can use the right mouse button and the cursor will be properly locked on the screen.
Go and build stuff!