There are several great posture building exercises.
One is to lay on your stomach on the floor (or on a pillow or something if this makes your back hurt) with your arms above your head. Then raise your pelvis off the floor and one leg and the opposite arm (pelvis, right leg, left arm or pelvis, left leg, right arm). This can be extended into Supermans once you're comfortable doing them, by simultaneously lifting both arms, both legs, and your pelvis at the same time.
Another is to simply do Planks. Lay on your stomach resting on your forearms. pull your body up into a strait line with only your forearms and toes touching the floor and hold that position as long as you feel comfortable, then lower yourself back down.
As Sparafusile stated, you can also do rows. One of the easiest ways to exercise the muscles exercised doing rows is to lay down on your stomach on a bench with dumbbells on either side and lift them up until your elbows are straight out to your sides and your elbows are bent at a 45 degree angle.
About.com also has a great article with more exercises to strengthen back muscles.
You should take into consideration your age as well, and any other conditions which may play a part in having a hunch in your back, such as scoliosis or damaged vertebrae or discs. If any exercise causes you extreme discomfort or pain, stop doing it immediately and talk to your doctor.