The Straight Leg Crunch is a great exercise for targeting your abdominal muscles.

Lie on the floor with your knees bent and feet flat on the floor. Bring one leg up so that it is straight and perpendicular to the other leg. Place both hands behind your head, tuck your chin into your chest, and lift up until you feel a stretch in your abdominal muscles. Lower back down with control to the starting position, repeat for desired reps, then switch legs and repeat for desired reps.

This exercise will target all three major muscle groups in the abdomen: rectus abdominus (upper abs), external obliques (side abs) and internal obliques (lower abs).