The Digi corporation manufactures over 70 different varieties of XBee modules with different antennas, power type, and capabilities. Parallax has evaluated and chosen the most useful of these XBee modules. Many of the XBee modules' functions such as addressing, power output, and baud rate for communication with a microprocessor can be configured through the free Digi X-CTU software utility or directly from your microcontroller through a terminal program. Parallax documentation demonstrates these features.
It’s easy to get started with Series 1 XBee modules, as they arrive configured to communicate right out of the box; Series 2 modules will require minor configuration before use. XBee modules sold by Parallax are compatible within their series; Series 1 will communicate with other Series 1 modules but not with a Series 2 module, and vice versa: XBee Series 1 and Series 2 are not cross-compatible. Major differences between Series 1 and Series 2 have been listed on Digi's site.
The range of the modules is determined largely by the power output (regular vs. Pro) and by the type of antenna mounted on the module. Digi provides a document summarizing the results of range tests for various modules: - XBee & XBee‐PRO OEM RF Module Antenna Considerations (.pdf)
Parallax offers the XBee modules at Digi recommended distributor prices, lower than you will find from most resellers. Parallax also offers different XBee Adapter Boards.
http://www.parallax.com/go/XBee