Ten million dollars of R&D distilled into a phenomenally capable chip that costs less than your morning latte: Nordic Semiconductor's nRF24L01 Single Chip Transceiver gives your Arduinos the power to talk to one another wirelessly across a 40 foot range. The SPI interface gives you control over a sophisticated radio modem which incorporates CRC error correction and handshaking to ensure your packets arrive uncorrupted. Each of the 126 user-selectable RF channels contains 6 uniquely-addressable data pipes. This opens the door for experimentation with mesh networks. There are four programmable power levels, and a standby power mode that consumes just 60 microwatts, enabling you to build long-lasting battery-powered wireless sensors. But you don't have to compromise on speed because this chip can transmit up to 2 megabits of data per second. It runs on 3.3 volts and accepts 3 volt or 5 volt logic inputs. This module has a built-in antenna. An extensive Arduino library and documentation is available at http://code.google.com/p/nrf24l01.