I've been out of the loop on ZigBee for a while now. Who has the most mature Zigbee stack? Is it Microchip, Chipcon, or Atmel?
Are there RF modules down to less than US$10 yet?
I recall there was some licensing issues with certain stacks a while back...
(This question was originally asked in 2009. As of 2012, I don't think Ember is a leading stack anymore. It seems that Texas Instruments has become the dominant chipset/stack... Although, their stack does seem to be locked to using IAR's terrible IDE which would make me think twice about using it.)