The pHduino project reports acidity levels over USB or an attached LCD -

The pHduino can operate using a LCD to display the pH and the temperature data like a pH meter. Also, you can control it using a computer by USB port.
The signal gain (slope) and the signal offset is adjusted manually by trimpots. The signal is compensated by a temperature sensor.
It seems inferior and a little bit more expensive compared with a pH meter bench instrument. However, it is interfaceable, programmable, expansible, and, the more important, open and free!
Again, it is not intent to be a high resolution analytical instrument to compete with a commercial pH meter. It is to be simple, easy to understand, easy to modify, and different!

Schematic, source code & more on Google Code [via Hack a Day]