Get that word toning out of your vocabulary, and off these damn boards! Seriously, though, you can't "tone" a muscle. I prefer decline crunches with weight. Even flat crunches with weight work well too. You can hang from a bar and pull your knees up until they're parallel with the ground, then spread them open and close them again. This takes immense strength in the abs. It gets tiresome quickly.
The ab ball or stabilization ball has always given me good results. But like any other movement your ab exercises need to be controlled and slow, it shouldnt be your intention to go in there and pump out 100's of crunches.
lets see - roman chair, hanging leg raises, cable crunch to name a few. i try and switch it up each w/o except for the hanging leg raises. i always hit those.
Decline weighted crunches, hanging leg raises, v-sits, cable crunches, ya those are some goood ones I switch things up every time i work my abs though.