Ultrasonic distance sensors
Ultrasonic sensor module US-100 
The ultrasonic sensor module us-100 is a convenient way for measuring distances from objects. This module has a lot of applications such as parking sensors, obstacle and terrain monitoring systems, industrial distance measurements, etc. It has a stable performance and high accuracy ranging from 2cm to 450cm. The module sends an ultrasonic signal, eight pulses of 40kHz square wave from the transmitter; the echo is then picked up by the receiver and outputs a waveform with a time period proportional to the distance. The connected microcontroller accepts the signal and performs necessary processing.
The US-100 can have a dip-switch. if it has, with the switch in place the US-100 will give a serial output.
Without it will give an analog output.
The US-100 has 2 groundpins. They need to be connected both.
Specifications:
Input voltage: 5V DC
Quiescent current: less than 2mA
Level output: 5V high
Level output: at the end of 0V
Induction angle: not more than 15 degrees
Detection distance: 2cm-450cm
Precision: up to 1mm
Dimensions: 4.4cm x 2.6cm x 1.4cm
Weight: 43g
some examples can be found here: http://arduinoinfo.mywikis.net/wiki/UltraSonicDistance
manual from Gizmo
manual
Ultrasonic sensor module HC-SR04 
The HC-SR04 has no digital output and only one groundpin.
Specifications:
Power Supply :+5V DC
Quiescent Current : <2mA
Working Current: 15mA
Effectual Angle: <15°
Ranging Distance : 2cm – 400 cm/1″ – 13ft
Resolution : 0.3 cm
Measuring Angle: 30 degree
Trigger Input Pulse width: 10uS
Dimension: 45mm x 20mm x 15mm
HC-SR04 User Guide
HC-sr04 user manual
complete-guide-for-ultrasonic-sensor-hc-sr04
Ultrasonic sensor module JSN-SR04T 
Ranging Distance: 20-450cm
Power Supply: 5V
Working Current: <8mA
Resolution: 1mm
Nauwkeurigheid: ± 10mm
Measuring Angle: <75 degree
Dimension: 42mm x 29mm x 12mm
Kabellengte: ~250cm
Can be used like the HC-SR04