A 433 Mhz receiver kit for under $10, this is designed for use with an Arduino and the wickedNode kit for a range up to 300' (we have tested it at more than that line of sight).
It is extremely simple to assemble and use, the kit includes the board, radio receiver, antenna, wire and headers.
Arduino not included.
Note: As of June 1, 2011, this product includes 2 6 pin long pin female headers. This provides improved access to the power, ground and analog pins of the Arduino.
Instructions for assembly are available at http://receivershield.wickeddevice.com/