For lower abs I've always liked pulse ups but there's a ton of others.
Incase you're unsure how to do a pulse up what you do is lay flat on your back with your hands at your sides, and bring your legs up straight then push (or "pulse") them up with your lower abs/hips so your bottom gets off the ground:
http://sixpackabsprogram.com/images/pulse_up.jpg
Then bring your bottom back to the ground but DON'T let your legs come back to the floor you want to keep them straight up in the air... keep them as straight as possible. When you do the pulse-up the momentum is going to make your legs want to go to floor. Engage the ab section so they don't!
Tip: Rather than lay your hands on the ground use your finger tips, some find this helps the motion. With the hands completely flat some were using their shoulders to help the movement.
For lower back AND abs I like the "Superman Banana"... start on your stomach with arms and legs raised (as if you were Superman) and hold it for three seconds, then roll to your back and hold your hands and legs up (as if you were a banana). Hold that for three seconds also then just keep alternating for how many ever reps (each reps is two movements, superman and banana) you think you require.
Those are without weights... with weights there's a ton for the lower back.
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