Drone with a thermal-camera. This data can be used for detecting water stress in fields and in crop phenotyping. The other useful application is for night time security surveillance for intruder detection and detection of predator activity at night 

Leaf Area Index (LAI) is a measure of the extent of leaf cover in crops. It correlates directly to biomass, which correlated to yield. In turn, NDVI is good indicator of LAI. So NDVI imagery can give a farmer a good idea of his expected yield, and where in his fields it is going to come from.

In the images below, the red areas correlated to low LAI, and they contrast with the green areas, which will provide better yield. 

NDVI can detect small changes in plant health over a few days, if you calibrate correctly. In this figure, the upper part shows an organic vineyard prior to treatment with organic plant nutrition. The lower part shows the result 3 days later. The second and fourth blocks were treated, the others left the same as control blocks. The NDVI clearly shows an improvement (more green) in the two treated blocks, confirming the efficacy of this plant nutrition product for the manufacturer. 

What does a section of fruit orchard look like in NDVI? The NDVI shows some variation here, but most of the area scanned is in good health.