Soil Moisture Meter
Handy little meter (tensiometer) that let's you measure the overall saturation level of soil and coco. It works by measuring the "pull" on the ceramic cone part on the bottom of the meter, this is roughly the same type of system plant roots use so it provides a good indication of what's actually happening down below in the growing medium.
There is a small digital display on top of the meter, this displays a reading of between 10 - 750 (millibar). If it says 10 your plants are drowning, if it's up near 750 then they're probably about to die of thirst, after you've used it for a while you get the hang of what's right for the type of growing medium you're using.
They're a bit on the expensive side if you wanted to use them one per pot, but for the bigger faster feeding plants they really help make sure you're on the right track with your feeding. Also very handy for setting up drip feed system, they can provide a good indictor of whether you've got your feeding schedule correctly set-up.