JeeNode (v2)
The JeeNode is a low-cost combination of radio and microcontroller board:

Think of the JeeNode as an Arduino + Xbee shield + Xbee, but smaller and less feature-laden than the XBee.
The JeeNode is based on an Atmel ATmega168 - the same as an Arduino - and was inspired by Modern Device's Really Bare Bones Board, which is like a minimal implementation of an Arduino. The USB communication interface has been removed and any USB-to-TTLSerial based on the FTDI232R chip is guaranteed to work as connection. Being compatible with Arduino boards such as the Duemilanove means that you can use the IDE to upload a sketch using any PC running Windows, Mac OS X, or Linux.
The radio is a HopeRF wireless module, which is available for the 433, 868, or 915 MHz frequency bands.
SPECIFICATIONS
See the JN2 Specs page for technical details, a list of parts, etc.
HOW TO GET IT
This version is no longer available, it has been replaced by the JN3.
You can still create your own boards with these CadSoft EAGLE design files:
http://jeefiles.equi4.com/jee-pcb-003.sch
http://jeefiles.equi4.com/jee-pcb-003.brd
Schematic as PDF: http://jeefiles.equi4.com/jee-pcb-003.pdf
Everything is provided as open source, both hardware and software.
HOW TO BUILD IT
Check out the JN2 Build page with step-by-step build instructions for the kit and board.
HOW TO USE IT
For some notes on how to test and use it all, visit the JN2 Tips page.
OTHER RESOURCES
The main documentation can be found at http://jeefiles.equi4.com/JeeNode-v2.pdf
All related weblog posts are listed at http://news.jeelabs.org/tag/jeenode/
Check out the forums at http://talk.jeelabs.net/ for further details and support.
SEE ALSO
The JeeLink is a pre-assembled SMD variant which plugs directly into USB.
The previous version was JN1.