The supine abdominal bridge is a great core exercise. It is relatively easy to perform and requires no fancy equipment. While the abs are the primary focus, the supine abdominal bridge also works the thigh and hip muscles (e.g. gluteus, hamstrings, quadriceps). They each allow the "bridging" of the hips to take place. Here is how this exercise is done:

Begin in the supine position (on you back) with your knees bent and feet flat on ground. Place the arms by your sides, palms face down. Next, "bridge upward" by pushing through the heels and lifting the hips off the ground. As the hips come up, contract your ab muscles for at least 2 or 3 seconds. The head should remain on the floor at all times. Slowly lower the hips and repeat the exercise.