The RIFFLE: An Open Source Datalogger. We are working to develop an open source hardware datalogger that can be placed in the field for extended deployments and measure common water parameters like temperature, conductivity, depth, and turbidity. Learn more at the Public Lab Riffle wiki.
The Coqui: using audio as a sensor data format.. We're also exploring using audio frequencies to convey sensor readings -- like a modem, but even simpler -- in order to make sensor readings more accessible for people (or to enable data transmission over ubiquitous audio jacks on PCs and smartphones). Read more about this idea on the Public Lab Coqui wiki.
A mailing list. Sign up for the plots-water-quality email list, and participate in discussions about low-cost, open source water quality monitoring tools and techniques.