Daintree, which makes expensive but powerful network analysis tools, offers a few useful Zigbee and wireless primers on their site.

• Daintree’s Getting Started guide.
• Here’s a paper about the benefits of mesh networking.
• An overview of Daintree’s commissioning tools.
• Jennic offers a good basic Zigbee primer.
• TI has a complicated Flash animation which covers a lot of ideas in the home and factory automation areas.
• RT’s article on using Zigbee to develop commercial products.
  

• OpenZB is an open source Zigbee toolset project, focussed on larger networks and the TinyOS platform.
• Using Zigbee with a Linux SLUG (as a traffic logger, etc.)