The best multiplayer game: Pixel Gun .

This game is a very good online game for any child and parent of any age.
The game comes with the ability to play a campaign, make servers (with and without the password), create a fun space where you can meet and chat, create castles and clans.
The game allows duels between teams, conquering the flag, choosing a pet, customizing the player with weapons and accessories, creating a space for each player. You can also create friends, invite and like them.
Pixel Gun 3D has two modes: a single-player Survival Mode and Multiplayer Mode.
First the single-player can be start with with a multi-level campaign and an interesting comic book story.
Into this story, you run around shooting mostly undead enemies like zombies and skeletons and earn points.
The Multiplayer Mode where you join others in a shoot-or-be-shot contest come with many servers and options to do that.
You can also create your own skin, update your clan castle and environment ( like pet house).
I could enumerate the options and features of this game but let you discover it.
I've spent hours playing this game and it's one of the fastest and does not require a lot of resources.
The game can be found here.

The Matter javascript for 2D collider.

The Matter.js is created with javascript and can be used like a 2D physics engine for the web development and game engine.
This javascript for web and mobile can be found on official website.
The development team tell us about the features of this tool:

• Rigid bodies
• Compound bodies
• Composite bodies
• Concave and convex hulls
• Physical properties (mass, area, density etc.)
• Restitution (elastic and inelastic collisions)
• Collisions (broad-phase, mid-phase and narrow-phase)
• Stable stacking and resting
• Conservation of momentum
• Friction and resistance
• Events
• Constraints
• Gravity
• Sleeping and static bodies
• Plugins
• Rounded corners (chamfering)
• Views (translate, zoom)
• Collision queries (raycasting, region tests)
• Time scaling (slow-mo, speed-up)
• Canvas renderer (supports vectors and textures)
• MatterTools for creating, testing and debugging worlds
• World state serialisation (requires resurrect.js)
• Cross-browser and Node.js support (Chrome, Firefox, Safari, IE8+)
• Mobile-compatible (touch, responsive)
• An original JavaScript physics implementation (not a port)

You can see one good example here.
How can be used ?
First, get a stable release and include the script in your web page and add the matter engine objects and features, see this example: