Hyperextensions

Hyperextensions works the lower back and the spinal erectors.

Position yourself face down across a hyperextension bench, with your heels hooked under the rear supports.
Cross your arms in front of your chest or behind your head and lower your upper body as far as it will go.
From this position, raise your body back up until slightly higher than parallel.
Don't go any farther than this or you can injure your back.

Repeat.
If your body weight isn't enough, you can try clasping a plate in your arms in front of your chest.