Unplug your project by adding a wireless sensor, $23.50, to your existing Arduino board. This one includes an easy to build transmitter node and receiver shield. I have used these in many home-control projects to wirelessly connect sensors to a central Arduino system. For example, I use this to keep track of room temperatures and humidity to tune the heating system in my 150 year old house.

The JeeNode v6 Kit, $22.50 (not pictured), is a transceiver package that includes an on-board Arduino compatible controller. Get at least two. I use these when I want wireless capability and an Arduino compatible controller in a small package. These units can send and receive and are a good choice when two-way communication is required. I have used them in lots of projects, the most recent was for a robot controller that displayed status on an LCD and sent commands to control robot movement.
Arduino Ethernet card, $58. Connecting Arduino to the internet enables your Arduino projects to communicate with the broader world over Ethernet and networks. You can remotely access your sensor data or take control of your Arduino’s actions from anywhere there is a network connection. If I could have one and only one Arduino board, this would be the one. Because of the endless variety of things to connect to, I have had more fun building applications for this board than with anything else.