My Watermark soil moisture sensor arrived from the US the other day, and I’m currently working on connecting it to the small computer that will do the measurements and put them online on this website. As far as I know it works by measuring the electrical resistance in some material inside the sensor, and this will be converted to a number describing the moisture of the soil. The price was $32.
This should work with owfs as well — though it’ll need calibration.
@Paul: Yes I believe so. I’ll be connecting it to the Moisture Meter board from hobby-boards.com. And then I need to make a control system for automatic watering, but that should be possible with owfs too.