Using a cheapo HC-SR04 Ultrasonic range finder, mounted to a servo this radar collects sensor information. The servo is set up for a range of 90 degrees. Everything is controlled by an Arduino UNO and fed back to a host PC running processing. The blips are then plotted on the screen and can show you possible obstacles. A lot like regular radar.
This is a great quick project. The perfect basis for an obstacle avoidance robot, or learning to combine the capabilities of the Arduino with Processing.
Breadboard layout, and Arduino sketch are available here.