JeeNode

JeeNode, manufactured by JeeLabs is an Arduino-compatible board; it's quite small and comes with a radio module which can be used to easily create a network of wireless devices.

Programming

Being based on the same Atmel chip, it's seen as an Arduino UNO and the usual Arduino IDE can be used to write and upload the sketches (the micro version uses a different chip, and requires a little more work to program).

Programming interfaces

Some versions have a FTDI interface, so you may also need an USB-BUB adapter (but I guess it can also be programmed using another Arduino, for example). Mine has an USB port on it, so it's not a problem.

Software

The JeeLib gives you access to the wireless module, I/O ports and so on.