If you want to learn all the best abdominal exercises, check out ab-core-and-stomach-exercises.com.  It has pictures and instructions for more than 100 abdominal exercises, as well as tips to lose weight and get a flat stomach.

If you want your abs to really burn try these killer ab exercises.
 
You can use 2 Bosu Balls to do planks.  Place your elbows on on Bosu Ball and your feet on another Bosu Ball and hold the plank position.  If you want a greater challenge, try lifting one leg off the Bosu Ball.
 

You can use cables to do many different ab exercises that really work the obliques.
Cables allow you to twist and rotate in ways that traditional ab machines cannot.  If you don't have access to a cable machine no worries, there are many other ways to work your oblique muscles.

If you've been doing regular ab crunches on the floor, maybe it's time to try doing your crunches on an incline.
 

The other day I added pictures of a few new ab exercises on a bosu ball.  They were Bosu Ball Crunches with the feet up, Bosu Ball Oblique crunches with the feet up, and Bosu Ball bicycle crunches.

Dumbbells can be used to work your abs.  Actually, you can use any type of weight to add some resistance to your abdominals.  You could also use medicine balls or weighted plates.
 
Some simple dumbbell exercises for the abs are weighted crunches, weighted russian twists, or dumbbell side bends.
 

Crunches on the Floor are probably the most popular ab exercise, but they are not the best exercise for everyone.
A major flaw with crunches on the floor is a lack of range of motion.  The abdominals work to curl the body forward  and decrease the arch in your back.  When you lie on the floor, the floor changes your posture and decreases the arch in your back.