I've done those pushing legs things once before, and what a slut they are! but you're right they work, thanks man.
Abs are all about diet.
However leg raises or captains chair as previously mentioned are the best imo
if you have a friend, have him stand over your head as you lay down so you can see his balls... just kidding, grab his ankles and lift your legs up and have up push your legs away as hard as possible, don't let ur feet ever touch the ground and do a few sets of those. really gets ur core a working :P