A few others I do:
1) Planck (I think thats spelled right): You get into whats basically a pushup position on the ground, but instead of your arms straight, you rest on your elbows. Make sure they're slightly under your body, and not too far forward. You can mix it up though, for different feels. Anyways, keep your back straight, and just stay in that position for about a minute. You can try to move your body around a bit to see what hits your abs more, but this is a good one for strengthening your entire core, as your body has to keep itself stabilized with your core.
2) Wood Chopper: Have a rope attached to the lower part of a cable machine. (I would usually use about 40 lbs or so I think, I don't really remember) Stand sideways-your side facing the cable machine, both hands holding onto the rope. Turn your body sideways at the waist (away from the cable machine) while pulling your arms up and away from the cable machine. If thats unclear, imagine standing with the cable machine directly to your right. Your hands are outstreched to your right side, both holding onto the rope. Then you turn at the waist towards your left, while pulling the rope up towards your left shoulder.
I've got about 3-4 other odd exercises, along with all of the normal ones that I do or have done, but the main thing is, keep your body off balance, always trying to stabilize itself. Those are the main motions that will really help add size and strength to your abs. When all of those motions become easy, then add weight to them, or do exercises where you use more than your body weight.