A MagPul Angled Forward Grip (AFG), that is.
I went ahead and picked one up this weekend as the rails are pretty rough on the hands and I have a dislike of vertical grips. Figured for $35 you can't really go wrong.
Interestingly, they had a rifle at the store that had one installed on it and my first impression was that I wasn't going to like it. You can see a ridge in the first two pictures midway down that is there for your fingers. I didn't like where it was because it ended up pretty much ON a finger rather than underneath it. However, that piece is adjustable and it comes with a flat plate in case you don't want it.
When I got home, I realized mine was flipped around and it turned out to be right where it was comfortable.
I'm very interested in taking it out for a spin. It feels just about perfect as far as a grip angle goes (it could use a little padding, in all reality, but I can deal with that). Grabbing it, I have a natural tendency to put my index finger in front of it (in between the AFG and the sling loop) and pull back. Don't know if that area will get too hot, so I'll just have to see.
Initial impression is that it's a comfortable grip. I wouldn't mind something for my thumb to grab a hold of as well. Some rail covers should help with that, but something molded might be even better.
And weapon lights are priced hilariously.