The Devantech SRF08 is the advanced model of Ultrasonic distance rangers available, with excellent width-of-field detection, I2C interface, and light-intensity sensor!
The MaxSonar-EZ1 Ultrasonic Range Finder is a tiny, easy-to-use ultrasonic ranging system that can be easily interface to any microcontroller.
The Parallax PING sensor is cool, but it's more useful when mounted on a servo so it can scan the environment.
The PING))) Ultrasonic Range Finder is a simple to use ultrasonic ranging system that can be easily interface to any microcontroller.
The Parallax TSL1401 Linescan Imaging Daughterboard provides one-dimensional sight to almost any microcontroller.
If you want a really small, compact sensor to detect obstacles coming at your device, the 2-direction sensor kit is just the thing!
Left, Middle, Right short range sensors in a small package.
The Bittybot Micro Line sensor is a convenient way to build edge or line sensors, especially for MicroSumos!
BittyBot's Nano line sensor kit is a tight and tidy .95" wide pair of sensors for detecting white/black differences, like a line on a surface. Ideal for line-followers or sumo edge detection!