Your foam roller is basically a giant rolling pin, and you can use it to strengthen and build your abs. Here's what to do.
Rollout From Knees
Aside from the ability to maintain a wider hand position, the biggest benefit with the foam roller is the opportunity to progress. The ab wheel or barbell variations may stop at the knees, but the unique padding of the foam roller lets you get up onto your feet.
You can also make it harder:
Rollout From Feet
And, yep, even harder.