IEEE Computer Society, "IEEE Standard for Information technology - Telecommunications and information exchange between systems - Local and metropolitan area networks - Specific requirements Part 15.4: Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Low-Rate Wireless Personal Area Networks (LR-WPANs); September 8 2006,
http://standards.ieee.org/getieee802/download/802.15.4-2006.pdf

ZigBee Alliance; ZigBee RF4CE,
http://www.zigbee.org/rf4ce

Atmel MCU Wireless Solutions,
http://www.atmel.com/products/zigbee/default.asp

Atmel's RF4CE stack "RF4Control",
http://www.atmel.com/rf4ce

ATmega128RFA1 - SoC
ATmega128RFA1 is an IEEE 802.15.4 compliant single-chip, combining the industry- leading AVR microcontroller and best-in-class 2.4GHz RF transceiver providing industry's highest RF performance for single-chip devices, with a link budget of 103.5dBm.
http://www.atmel.com/dyn/products/product_card.asp?part_id=4692

AT86RF212
The AT86RF212 is a low-power, low-voltage 700/800/900 MHz transceiver specially designed for low-cost IEEE 802.15.4, ZigBee, 6LoWPAN and high data rate ISM applications.
http://www.atmel.com/dyn/products/product_card.asp?part_id=4349

AT86RF231
The AT86RF231 is a feature rich, low-power 2.4 GHz transceiver designed for industrial and consumer IEEE 802.15.4, ZigBee, 6LoWPAN and high data rate 2.4 GHz ISM band applications. The radio transceiver is a true SPI-to-antenna solution. All RF-critical components except the antenna, crystal and de-coupling capacitors are integrated on-chip.
http://www.atmel.com/dyn/products/product_card.asp?part_id=4338

AVR2025: IEEE 802.15.4 MAC Software Package for AVR Z-Link
http://www.atmel.com/dyn/products/tools_card.asp?tool_id=4373&family_id=676