I’ve just been browsing a little (waiting for the coffee to kick in!) and come across some SR04 distance sensor modules at Hobby Components. These are ridiculously cheap at £1.75 (inc VAT) and are very close in price to those available from China. I use one of these in my Picorder and several other people have had success using them. It doesn’t take much programming and the sensor sends a digital signal (so no mucking about with analog-to-digital converters).
These are great fun if you need to measure distance or be alerted on proximity. The sensor does its stuff by sending out ultrasonic pulses (like sonar) and then reading how long it takes for the ‘pings’ to come back.
I don’t have any affiliation or agreement with Hobby Components, I’m just passing on deals when I find them, whoever I find them with.