BrainBlitz.org

BoxCAD 2.0 is coming... sometime. Whenever it DOES come, if you like the current version, 2.0 will knock your socks off. Full blown game creation with key controllers, basic scripting, and more. All dumpable to AS3 box2d code, or xml for whatever physics engine you're using. So bookmark this is or something, and check back later.

Features:
Add keyboard controlled forces to objects - Particle Engine Jet Motor Visual - Fx/Motion blur - Full Screen Mode - Open API if you want to make your own tools - Dump to AS3.0 (box2d 2.0 code) - Dump to xml, for import into other physics engines - Publishing tools, so you can add a slimmed down version of your creations to your own site - Lots of tools and behavioural changes to facilitate creation of truely complex machines (hard to do with the current version).

BoxCAD is a 2D rigid body drafting and simulation program, targeting the intelligent-but-bored and game developers.
The goal of BoxCAD is to provide a platform for the accurate creation of complex virtual machinery for entertainment. No guarantees are made to the accuracy or reliability of the software. It is not recommended for anything other than entertainment purposes.
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Creating and Manipulating Objects

Create objects by using the command buttons on the tool window. Bodies (circles, nodes, polygons, etc) can be selected and manipulated after creation. To select an object, click inside it. To move and rotate objects, drag and roll the mouse-wheel, respectively. If the simulation is running, the body being manipulated will respond to the mouse as a large force for realistic interaction. If the simulation is not running, bodies will respond directly to the mouse. Be aware, moving a body in Edit mode that is constrained to other bodies will not modify the constraints. Upon restarting the simulation, restraints will attempt to resolve attached bodies to their initial solution. If you are not careful, it is possible to create a scene with no solution (conflicting constraints), which will behave unpredictably.
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Editing Objects

BoxCAD generally uses Noun/Verb operation. First you select objects, then you use the command. To select an object, click inside it. To add multiple objects to the selection set, hold down shift as you click. Some commands only operate on one object (i.e. mouse-wheel rotate) - if there are multiple objects selected, such commands will operate on the first object selected. Other commands (i.e. Delete Objects) will operate on the entire selection set.

Keyboard Controls:

Esc: Cancel Command
c: clear selection
r: repeast last command
u: undo last object
Space: Start/Stop simulation

Commands:

Start/Stop simulation [space bar]
Toggles the running state of the physics engine.

Circle
Click to set center point, then click again to define radius.

Rectangle
Click to set one corner, then click again to define opposite corner.

Polygon
Click to set each consecutive vertex. The polygon object is limited to 8 vertices, and must be convex (must not have any "innies").

Node
Click once to set a node. A node is a small, static circle object.

Particles
Click and hold to create particles. Particles are small circle objects.

Distance Joint
Click inside first body at desired anchor point. Then click inside second body at desired anchor point. Use Snapping for accurate anchor placement.

Revolute Joint
To pin two objects with a revolute joint, make them overlap, then click at desired anchor point. To pin an object to world, simply make sure it is not overlapping another object.

Motor Joint
Same as a revolute joint, except that it induces a constant, opposing torque in each body.

Cancel (esc)
Will cancel any command at any point.

Clr Selection [c]
Empties the selection set.

Group Selected
Assigns selected objects to a group. Objects in a group will not react to each other.

Delete Object
Will delete every object in the selection set, and any constraints attached to those objects.

Undo Last Body
Deletes the last object created.

Repeat last command [r]
Pressing 'r' will repeat most previous commands.

Dump Code
Generates ActionScript 3.0 code.

Gravity Multiplier
Adjusts gravity force on a quadratic scale.

Sim. Iterations
How many times the physics engine is stepped per frame.

Time Step
The inverse of the time step computed by physics engine. A larger value means more accurate simulation, but will run slower.

Snap to Grid
Causes every operation to round to the nearest 5th unit, for drawing accuracy.

Snap to Obj. Cntr.
Causes every operation to execute at the center of the body the mouse is currently over. Particularly useful for creating balanced wheels.