Like skirt steaks? You'll love flap steak. It can be used in most skirt steak (and many flank steak) recipes, but it tends to be a more tender cut than the skirt steak. Cut from the bottom sirloin, flap meat is what you want if you want spectacular fajitas.
We recommend reading the Serious Eats article for a good background on preparing the steak if you are unfamiliar with it. Please note the point about this steak being one of the few beef cuts that is better cooked to medium rather than rare.