About ZigBee

Hi, i want to use zigbee: i know only freescale ZigBee and i'm looking for better alternatives...For me it is very important (in order):

1. Open source ZigBee stack (a stack with obfuscated source code is very difficult to use)
2. System on chip solution (micro 8bit+transceiver) (better at 868MHz)
3. to be used in very low power solution
4. low cost Any suggestion? Thanks in advance