The 8051 microcontroller is the grandfather of embedded microcontrollers.
It was designed by Intel in 1980 and although there were microcontrollers around before then, the 8051 has been the most successful.
8051 processors are generally a little outdated as a processor in their own right, but the 8051 core is commonly used in specialist silicon such as Nordic RFchips.
The Bluefruit team used a 8051-based controller in a recent Digital DJ Mixing Desk project. Although we used a Microchip PIC for the central processor, we used a Texas Instruments TUSB3200 for the USB audio. This USB streaming controller has a 8052 core.
The 8501 core featured again in a recent wireless temperature sensor project where we used the Chipcon CC1010 controller.