A graphical user environment cum multi tasking operating system released in 1981 for the amazing PERQ series of workstations. It had windows, worked with a mouse or lightpen, and did WYSIWYG layouts. You could develop Accent programmes in C, Pascal, or SPICE LISP.
The most interesting thing, however, is the fact that the Mach Microkernel directly descends from Accent. The Mach Kernel is the bit that BSD Unix (including FreeBSD, OpenBSD, NetBSD), NeXTStep and OpenStep, and consequently Darwin, the core of Mac OS X are built on.