Here is The Nooliberry, a RaspberryPi GPIO extension board with 802.15.4 radio. It's based on the Atmega 1281 and comes with an AT86RF230 radio chip.
It's currently supported in Contiki, and you can turn your RPi into a standalone 6LoWPAN Border Router solution for Internet of Things applications, using 6LBR.
6LBR and The Nooliberry are both open-source.
The Nooliberry wiki points to where you can order these cards.
Of course, this board could be used for a ton of other applications which need a 802.15.4 interface.