Maggie's Farm

Planks are isometric core exercises.  While many or most exercises require or produce a strong core,  planks are pure. They are good to throw into calisthenic circuits. (Crunches are generally not a great idea). Another good core exercise is one-handed kettlebell swings.  I also do Roman Chair leg lifts weekly, the back raise thing, one-handed Farmer's Walks, and mountain-climbers. 

Contrary to common belief, core exercises do not give you 6-pack abs. 6-pack abs are mainly a product of low body fat. 

The key to effective planks is to make them isometrically-active. That means pulling your tummy in and tightening your glutes during the time of the plank. That's a rigid plank, not a relaxed plank.