Not using many at this point but a couple of my favorites are:
Arccos Caddie – used in conjunction with Cobra Golf to help with golf club yardage and instruction. Tracks distances with each club and rounds of golf – where you hit each shot and lets you know where your strengths and weaknesses are.
USGA Rules of Golf – with the new rules in effect since January of 2019, a lot of questions come up. Stroke and Distance local rule, penalty areas and the new way to drop the ball are always discussed with my members.