The Parallax PING sensor is cool, but it's more useful when mounted on a servo so it can scan the environment.
The Phidgets Sensor Cable 350 is used for the analog input on the Phidget Interface Kit 8/8/8.
Simple breakout board for the SparkFun MQ-3, MQ-4, MQ-6 and MQ-7 gas sensors.
We've had an eye on this Vishay sensor for a while - smart, tidy, and small. Thanks to SparkFun for breaking it out so you can use this to measure ambient light, and physical proximity in a single sensor!