If you're looking for a challenging lower ab exercise, try 1 leg piques with a stability ball.
The stability ball is a great tool for core training and abdominal exercises.
Because the stability ball is unstable it will challenge your core muscles to a greater degree and increase your stability.
Learn all the best abdominal exercises.

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.

The Stability ball can be used in so many ways to increase your core strength.  In addition to crunches and piques, you can also do many different plank variations using the stability ball.
 
Try this hard core exercise on the stability ball: plank forward rolls or try plank side to side rolls on the stability ball.
 

The plank and its many variations are best abdominal exercises for women and men.  Combine the plank with a stability ball and you've got the makings of a great core workout.
 
Try 1 leg planks with the hands on a stability ball or try them with the feet on a stability 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.

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.

There are many Ab Workout Mistakes, but the #1 ab workout mistake is trying to spot reduce with ab exercises.
Because of the media people keep trying to get rid of belly fat simply by doing ab exercises.  But spot reduction is the #1 Ab exercise myth.
If you want to lose belly fat, you need to take a comprehensive approach and focus on cardio, dieting, and weight training.

If you are not varying your movements, this is an ab workout mistake.
The spine is designed to move in multiple directions, forwards and backwards, side bend left and right, and rotate left and right.
If you are only exercising your abdominals in a front to back motion then your spine is not strong in all directions.
Add a variety of movements to have a balanced spine.

Hello fellow fitfriends.
My name is Charles Inniss and I am a Physical Therapist and Personal Trainer.  I will do my best to contribute worthwhile health and fitness information so that you can reach your goals.
My website http://www.Ab-Core-and-Stomach-Exercises.com aka All-About-Abs.com has tons of pictures of abdominal exercises, free ab workouts, and weight loss tips.
I hope that I will be able to help you in some way.
Yours in Health,
Dr. Charles