Radiocrafts offers 3 “ZigBee-ready” modules. The modules are small, contain a CC2420 and one of three Atmel AVR cpu's and are available with or without an on-board antenna. The product flyer for the series is here. And the datasheet is here.
The price for the largest module is about 100 euro in small numbers. GREATech, and possibly others, have the modules in stock. The module is delivered with zigbee stack (from Figure 8) and the royalty fee is included in the module. I am unclear if this includes the source code. WinAVR is explicitly mentioned as a recommended development tool.
The AVR + CC2420 combination is used in the Chipcon Zigbee developer kit as well.
And by the way..
Both Chipcon and Radiocrafts are located in Oslo, Norway. The AVR architecture was developed by two students of the Norwegian Institute of Technology. It seems many good things come from Norway..