As others have stated, the Parallax PING))) is just awesome.
Here's a link to a place where I posted my driver for the PING))) sensor.
Only needs 1 pin/sensor, and have really narrow angle acceptance ranges (narrow beam) which is great for finding objects. Not to mention, the price isn't half bad either.
Oh, and if you don't mind my asking... what are you using this sensor for?
-q
p.s. If anyone is interested, I have code that will run four PING)))-type ultrasonic sensors at once (simultaneously/asynchronously) that just needs a little clean up to be post worthy. Just let me know.