The low-power XBee 802.15.4 and extended-range XBee-PRO 802.15.4 use the IEEE 802.15.4 networking protocol for fast point-to-multipoint or peer-to-peer networking.
The XBee series of 2.4GHz communication modules take the the 802.15.4 stack (the core of Zigbee) and lays a very useful serial command set on top of it, making them very popular with a huge user community. If you want to take any sort of serial data and beam it "out there" wirelessly, this is the critter for you! Did you know they also talk point-to-point or in a multi-network mode? Swarmbots - woo-woo!
This particular model is the "XBee 802.15.4" (formerly Series 1) part XB24-ACI-001, and features: