What is a lean cut of beef (steak) to use in low calorie recipes?
According to the Mayo Clinic, the leanest cuts are:


Eye of round roast or steak 
Sirloin tip side steak 
Top round roast and steak
Bottom round roast and steak
Top sirloin steak


However, grain/corn fed beef may have more marbling (more fat embedded) so if you're going for low-cal, you might want to consider buying naturally raised/grass fed beef.

Personally, I prefer to find a meat source that has leaner beef in general and buy Skirt Steak or Clodhammer (rotator cuff) for their taste, tenderness, and reasonable price.