One of the best ways to work the rectus abdominis is actually tricep pushdowns, if you do them like this. Anyone who's ever done them this way ends up very sore from it.
I actually learned this from Franco Columbu. He hated training his abs and simply figured out a way to train them with pushdowns.
They're basically leaning triceps pushdowns. To do these, step away from the cable a few feet rather than being close to it. From there, push out and down into your abs.
If you do this correctly, you'll create tremendous tension through the abs and force them to stabilize really hard. Plus you're working pipes while doing it, and big pipes get about as much attention as rock-hard abs.