I installed jeenodes to every room in our home, to meter humidity, light and temperature.
One potential and very interesting use is metering moisture: plants and water pipes (for example underneath of the washing machine).
Using resistive metering (either with voltage divider or Wheatstone's bridge) is really easy to implement. Unfortunately it's difficult to meter moisture in plants with resistive metering because soil and impurities change metering really much.
For example AD7746 (two channel, datasheet at http://www.analog.com/static/imported-files/datasheets/AD77457746.pdf) looks like really good candidate for capacitive metering. AD7746 costs only ~$6/pcs from Analog Devices, so building it as a plug is not really expensive (yes, VAT, assembly, manufacturing boards increases the price, unfortunately). Many breakout boards found from internet (for example from Sparkfun) are ~$20, which sounds way too much.
AD7745 (same but with only one channel) is used with arduino for example in here: http://wiki.mech.uwa.edu.au/index.php/Mechatronics:SoilSensorDesign&Specifications .