Other terms:
Wireless Obstacle Avoidance System
Reverse / Backup Sensor
Parking Sensor
Backup Systems

Best sensitivity according to 2004 ConsumerReports (CR) test:
Ultrasonic
Grote Obstacle Detection System $275
EchoMaster EM-PV (bumper moounted sensors) $275
Rostra Obstacle Sensing System 250-1594 $350   only higher-rated model not affected by inclement weather.

Dolphin SonarStep (same as EchoMaster sonar step): $100-200
(Not tested by CR) Mounts on 1¼ or 2" receiver hitch
For use with hitches over 15" from road surface.
Wireless sensor plugs into cigarette lighter. Give 2 levels of beep depending on distance.

Quotes from a Hummer forum.
I have the Dolphin and like it - when it works. The problem is that you need to keep it clean in order for it not to beep constantly. Being on the trailer hitch, it tends to get dirty easily.

Crimestopper "Backstopper Rear Vehicle Impact Warning System".
Dash Mounted LED Panel with Numeric Distance and Colored LED.
3 Distinct Beep Tones Notify You of the Distance to an Object.

Microwave-based sensor systems are less sensitive as a group. They also donŐt warn the driver unless the vehicle or object behind it is moving.

Cameras:
I tried the Walmart camera and wasn't happy with it and returned it.

Audiovox RVMPKG3 RVMPKG2 at $400 was one of the best cameras tested by CR but was difficult to install.

Virtual Reality Video Labs - VRBCS300W $100-$149 (wireless transmition) HitchCAM Framecam HCFC-1B HCFC-1C $800 got the top rating.