Before Windows Vista became a commercial reality (and a controversial one at that), there was — the codename for what was supposed to be Microsoft’s revolutionary successor to Windows XP. Development spanned from 2001 to 2006, and during those years, many builds leaked with experimental, half-finished features. The Windows Longhorn Simulator is a fan-made web application that lets you explore the look, feel, and promise of those early builds without installing unstable OS images or virtual machines.