compiler: http://sdcc.sourceforge.net/

C51 development tool: http://www.keil.com/c51/

51 series Chinese forum and other microchips: http://www.51hei.com/mcu/

Learning: MCU programming with 8051 chips: http://www.mikroe.com/products/view/267/architecture-and-programming-of-8051-mcu-s/

OpenNI : http://www.openni.org/
The OpenNI organization is an industry-led, not-for-profit organization formed to certify and promote the compatibility and interoperability of Natural Interaction (NI) devices, applications and middleware.