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.
 
These are killer stomach exercises, but if you don't have a bosu ball, don't worry visit my site to see pictures of over 100 of the best ab exercises.