The Atmel 89C52 has "Idle Mode" in which the clock to the CPU is turned off but interrupts, timers and uart still function. You get out of idle mode by interrupt or hardware reset. "Power Down Mode" turns the clock off completely, and you're stuck until you get a hardware reset. Atmel's TS8xC51 chip will wake from power down mode with an external interrupt, as will Dallas and Winbond parts.