Rc(1) is the Unix v10/Plan 9 shell[1] written by Tom Duff[2], of Duff's device[3] fame. Rc(1) is of course available in Unix as well, I use the port from plan9port[4].

[1] http://doc.cat-v.org/plan_9/4th_edition/papers/rc

[2] http://en.wikipedia.org/wiki/Tom_Duff

[3] http://en.wikipedia.org/wiki/Duff%27s_device

[4] http://swtch.com/plan9port/