Your World of Text is an infinite grid of text editable by any visitor. The changes made by other people appear on your screen as they happen. Everyone starts in the same place, but you can scroll through the world using your mouse. [more inside]
posted by grobstein on Aug 14, 2009 - 85 comments

Minecraft is a multiplayer block-based sandbox game. It's built in Java, so no installation is required (except for Java). Reddit already has their own server; why don't we? [more inside]
posted by archagon on Jun 9, 2009 - 28 comments

An Interactive Space Simulator "Smash planets together, introduce rogue stars, and build new worlds from spinning discs of debris. Fire a moon into a planet or destroy everything you've created with a super massive black hole. You can simulate and interact with our solar system: the 8 planets,160+ moons, and hundereds of asteroids, the nearest 1000 stars to our Sun, and our local group of galaxies." [31Mb, Windows only, sorry, but see inside for similar Mac and Linux apps] [more inside]
posted by stavrosthewonderchicken on Jul 11, 2008 - 27 comments

Let's have some physics phun! [more inside]
posted by flatluigi on Feb 20, 2008 - 26 comments

Flash Sunday: Particle-based (explosive) sandbox - another fun toy to play with! Besides the classic earth, water, fire, and plants, this version has air pressure-based physics for wind effects. A mesmerizing tinker-toy. (requires java) (previously) (via)
posted by anthill on Jun 2, 2007 - 21 comments