The SBR P*grip isn't made just for SBRs, of course, but offers a lightweight, slim-profile grip that will fit all mil-spec lower receivers and fosters solid purchase with two different grip profiles. The rear of the grip has wide serrations to keep things stable in the hand, and the grip face has light ovals to keep a firm grip without grating your mitts.
The grips have a unique, patent-pending palmswell in the middle that makes them feel a lot larger than they are. I've got reasonably large hands, and this diminutive grip fits me just fine (side note: You know you've got good muscle memory when you pick up an AR pistol grip that is not attached to any firearm and your trigger finger still indexes properly...).
MSRP: $19.99, available in black or flat dark earth.