One of my "new" exercises is a cable crunch. Sure it looks like some kind of medieval torture but you can really feel it working:
1. Take a mat and place it at the base of your cable machine
2. Choose whatever grip is most comfortable for you - I like the ropes
3. While kneeling reach up and grab the grip - pull the cables down so that your forearms are on either side of your head
4. Contract your abs, round your back, *think curl* and pull your elbows down towards your knees
5. Make sure you keep your abs contracted and breathe (exhale when you're going down)
6. Release and bring the cable back up in a controlled manner (inhale)
Perhaps it would be useful to see a demonstration. I looked up and down and I found a great animation demonstration here.
Trust me, this is an awesome exercise!