98ish

A simulated operating system in a nostalgic style. Created by the Brandon Taylor and Cameron De Robertis, the fresh minds behind Simul8-OS

Features

Draggable, resizeable, windows with maximized and minimized states
Tree data structure for filesystem, including live search of files/directories and built-in text editor
Several interactive applets (Tetris, Minesweeper, YouTube client, etc.)
Socket.io-based instant messaging applet with support for sharing YouTube links and multi-user chatroom
Unlimited applet multitasking