best one i have heard of is frog kicks.

you hang from a bar (best if you have someone to hold you and keep you from swinging at first or get on one of those circuit stations designed for this exercise if you have access to one) and simply raise your knees to your chest as many times as you can.

great for your lower abs and lower back.

i know people say there is no difference between lower/upper ab exercises but i feel the work concentrated in the upper and lower when i do the exercises so poop on them. i know that you cant get one working without the other, but the logic i heard behind this is that your lower abs are naturally weaker and exhuast more quickly than your upper abs. so its best to do exercises that target this region and then move to ones that target the upper.

where's my beer?