Captain’s Chair Knee Raise
You know that thing in the gym that you climb into and support yourself in on your forearms with your back pressed against a pad as you do knee raises? That’s called a captain’s chair and it’s mighty effective for hitting the rectus abdominis.

No doubt you’ve done these before, but keep in mind not to swing your legs or jerk your hips up and forward with each rep. Let your abs do the work, raising your heels as high as you can—above your head if possible. This aims at the lower rectus abdomens.

Alternate Knee Crunch
Lie on your back on the floor with your legs tucked up so that your thighs are perpendicular with the floor and your lower legs parallel with it. Clasp your fingers behind your head or cup the back of your head and alternate pulling each elbow to the opposite knee. Concentrate on curling your torso forward with each rep and returning to the start position with the opposite movement. This hits the upper rectus abdominus, obliques and intercostals.

Flutter Kick
Lie on your back on the floor with your arms down at your sides and the backs of your hands tucked under your hips. Alternately raise one leg, then the other, from a position of an inch off the floor to about eight inches off the floor. You’re essentially doing with your legs what you do when you’re swimming. It’s a quick but controlled movement. This targets the lower rectus abdomens.

One-Arm Kettlebell Crunch
Lie on your back on the floor with a kettlebell or dumbbell in one hand. Ten to twenty pounds should do the trick to start. Extend the arm with the weight so that it’s perpendicular with the floor as you keep your other arm to your side. Bring both feet in so that they’re flat on the floor. Push the weight as high as you can using just the muscles of your upper torso. This will work your serratus and intercostals.

Weighted Lying Knee-In
Sit on a bench with a 10-20 pound dumbbell (to start) placed on one end between your feet. Press your feet together and lie back, holding the end of the bench above your head. Raise the dumbbell with your feet by pulling your knees in towards your torso. Extend your legs until they’re straight and withdraw them again. This is a great exercise for your lower rectus abdomens and your intercostals.

Weighted Twisting Roman Chair Sit-Up
Hold a plate either against your chest or behind your head as you perform Roman Chair sit-ups with a twist of the shoulder towards its opposite knee on each rep.

Gut Buster
Get on all fours with your back straight, but your stomach relaxed. Suck your stomach in as far as it will go while curling your spine. Hold this position for 30 seconds with minimal breathing.
Source: http://www.muscleandfitness.com/