This module adds support for 6LoWPAN over 802.15.4 to the Raspberry Pi. With this, you can very cheaply turn a Raspberry Pi into a 6LoWPAN border router for your Internet of Things.
The module plugs directly onto pins 15-26 of the RPi's P1 header, leaving the remaining pins free for other uses.
Its intended use is to turn an RPi into a border router for a 6LoWPAN network, but it has other uses as well. You can skip the 802.15.4 and just use it as a basic 2.4GHz radio, and you can even hook it up to an Arduino if you want, or anything else with an SPI port and 4 GPIOs.
Linux recently has usable 6LoWPAN and 802.15.4 implementations, as well as a driver for this module.