When thinking about ab workouts, the default exercise is a crunch. They are easy to do, have several variations and get the job done. But sometimes, you just don't feel like doing another crunch. It's a common misconception that you can only work your abs with crunches.
You can use Pilates and Yoga to tone the abs or core section of your body with exercises like plank, leg circles and balance poses. If none of this is what you are thinking for working your abs, then check out this great slideshow that Fit Sugar put together: 5 Crunchless Ab Exercises .
You might be surprised how many of those exercises you've seen and done before but somehow they dropped out of your exercise program.
Always be on the lookout for new moves to keep your workout fresh and never be afraid to go back to old exercises that work!