Favorite Core Exercises?

THIS is a great ab workout, I'm enjoying adding it in a couple times a week, and I've noticed a difference in my training. 

When I did pole vault, we did a core exercise like this:  lie on the floor with your head next to a well-secured vertical pole (as part of a weight machine, dip/pull up station, etc).  Grip the pole with your hands just about 10-12" above the floor.  Pull with your arms, and raise your feet to a vertical position... BUT, don't bend at the waist at all.  Only bend at the shoulders and neck, so your head stays on the floor, but your torso, waist, and legs are all in a straight line.  So your feet will come off the ground about the same time as your butt does. 

If you can't do this once, bend at the waist on the way up, then get your body in a line pointing straight up, and slowly lower yourself.

Doing this right, knocking out a few reps is a fantastic core workout.

Good core work on the stability ball for me.  Pikes on the ball.  Pushups on the ball (either with my hands on it or my feet), knee drives, roll-outs, all kinds of variations.  Core killer for me is hands on the floor (like you're in a push-up), feet up on the ball so that just your laces are on it, not your shins.  Lift one leg up and use the other that is on the ball to drive your knee forward and back, or life one foot up and do push-ups, all kinds of good stuff.

And someone else mentioned, don't forget your back!  Huge for core!

I like the TGU! 

My fav is the single leg squat with one foot on a BOSU Ball.  Rough on the core and great for the legs and hips