Yardening is not a precise activity requiring close tolerance in measuring what we are doing.  At the same time having some idea of the parameters of something are often interesting and sometimes helpful in getting the job done more quickly and with more success.  

Thermometer - Knowing when first frost may occur is handy.  Plants don't grow much in temps over 80 degrees so knowing when that happens helps to understand why your plants are just sitting there.

Water meter -  This tool is handy indoors or outdoors.  It tells you when you plant needs water.

Light Meters -  These are