JS/UIX is a UN*X-like OS for standard web-browsers, written entirely in JavaScript (no plug-ins used). It comprises a virtual machine, shell, virtual file-system, process-management, and brings its own terminal with screen- and keyboard-mapping.

Amazing, even includes a simple (but working) vi-clone. Must be fun to try to port small UN*X command line applications (like fortune), especially with the help of a C-to-JS-compiler, if there is such a tool (a quick googling was fruitless).

Another stab at the same idea is JUnix. JS/UIX seems to be more complete (I could be wrong here, the JUnix documentation is written in Russian), but JUnix provides /dev/cookie which is certainly a good thing to have. 4KB could be enough to store a simple ~/.profile.