The Magpul RVG Vertical Grips are shipping. You may remember these from SHOT Show 2010.
These have some pretty nice features for such an affordable grip. The RVG comes with a short section of rail so the RVG can be mounted to your MOE handguard. It is designed to be used comfortably with a thumb forward grip.
I like that the design is hollow so the grip to can be trimmed to whatever length the shooter desires. Some shooters, who use the RVG as a handstop or index point, will only need a small stub of a grip. A few moments with a hack saw and some sand paper will allow the user to completely customize the length of the RVG.
I have only seen these available in black, but I suspect that other colors will be available soon.