Good for the abs.

A yoga pose I've always found to be challenging is the elevated sphinx pose. Giving one a workout similar to plank, elevated sphinx pose is basically when one takes plank pose, but on the forearms. When in elevated sphinx pose, be sure to keep your abs tight, and push through your heels toward the back of your mat. This will help ensure your spine stays straight.

Elevated sphinx pose is great for upper arm/shoulder strength, and is great for reaching one's goal of getting killer abs.