Recently bought the AVR Raven 2.4 GHz Evaluation Kit by Atmel. A development Kit for 802.15.4, Zigbee and 6LoWPAN networks. I contains one USB dongle which is basically meant to hook up your computer to one of the mentioned networked and two AVR Raven boards. They are battery powered have a 2.4 transceiver, display, small joystick, temperature sensor JTAG and ISP for the start. Of course there are also some IOs to connect external sensors. I found this hardware having a look at the IPv6 enabled contiki OS. I think I will spend some time with this hardware exploring sensor network technology :-) By the way, I think this development kit is pretty cheap. You can get all the stuff for less than 120 EUR!