US-100 modules are an alternative to the more common HC-SR04 ultrasonic distance modules. The absolute difference in price is negligible (US-100 are available from £2.32 as of mid-2016), while the US-100 increases accuracy and precision with no real negatives.
The library on GitHub below is for use with these US-100 ultrasonic distance modules when being used in serial mode (rather than the trigger/echo mode), examples are included. Full details are in the README file.