The plank is simple: lie on your front, then raise yourself up onto your elbows and make your body rigid, so that you are straight - like a plank. Remaining in this position requires your abs to remain under tension.
Outstretched Arms Plank
I invented this one, inspired by Jack Lalanne Push Ups, which I noticed hurt my abs as much as my chest and shoulders. Get into the basic plank position, then raise yourself up onto your hands instead of your elbows and walk your hands forward as far as you can whilst still maintaining your plank-like rigidity.