Your core strength is crucial to your overall fitness and strength level. Many people begin exercise routines and can figure out basic exercises for arms and legs, but few beginners know effective exercises for their core muscles. Don't be one of the uninformed exercisers who doing crunches endlessly and then get discouraged when they don't see the flat stomach and toned back they are aiming for.
Ab rollers have become increasingly popular for a quick, simple core workout. You simply hold on to the handles while kneeling, roll the wheel forward on the floor until your back and abs are flat (similar to a plank position). From here you can hold the position to increase stabilizer muscle focus, or roll back towards your knees. Straight rolling targets the upper and lower abdominal muscles as well as the upper and lower back muscles. To hit your obliques, roll in an arch to either side.
Towers and arm slings are also great for a comprehensive abdominal workout. The advantage of the arm slings is that they allow you to target the lower abdominal muscles which are a problem area for many people and they don't get hit hard with traditional crunches. Using your arms to support your body weight in a vertical position also works out your shoulder stabilizer muscles as well as your chest and upper back. Lift your legs at different angles to target different muscle groups.