Ever since I got into the AR-15 scene, I've seen a LOT of people expressing great concern about wanting their upper and lower receivers to fit together as tightly as possible. While some folks take this to the extreme of buying upper receivers until they find one that will fit tightly with their lower, I find that it's not THAT critical. However, it's a bit annoying to have a rifle where the upper and lower shift noticeably.
Enter the Accu-Wedge. I'm not sure whether it's rubber or some kind of polymer, but it's a spongy bit of red stuff that goes into the spot directly beneath the rear takedown pin on your AR. You insert the accu-wedge into the slot, then put the upper down on top of it, then put the whole thing together. I've installed it on one of my rifles with a very loose fit, and it went from very loose to only shifting a little.
The big thing which convinced me to give it a try was the price. From Midway, with a C&R discount, it's only $3ish. Nothing else that cheap will make as big an improvement in your rifle's ergonomics.
I will say, though that there are many people who should NOT invest in the Accu-Wedge. Namely, my fellow Californians who use pinned-magazine rifles. Of necessity, the accu-wedge makes the fit between the upper and lower receivers much tighter. This would translate to extra stress on the takedown pin when you're flipping the top open to load it. However, in a gripless, detachable-magazine configuration like mine, there's no reason not to use an Accu-Wedge to tighten up your rifle a little.
On the other extreme, we have people who want the utmost in tight fits on their receiver. For them, the best solution is the fancy tension pin solution from JP Precision. Their system fits into the takedown pin hole, then you use a tool to screw it in place - at which point it sort of mushrooms in the hole. You won't be removing it without an allen wrench, that's for sure.
Considering that the accu-wedge comes with no installation instructions, I felt like doing my own tutorial on what it does and where it fits. While I imagine that most folks who order one will figure it out in short order, I think these pics will help folks understand why they might want one on their rifle.
It is worth noting that once you have installed the accu-wedge, it will be considerably harder to get your AR's rear takedown pin in place, due to the extra tension. I personally place the rifle vertically between my knees (barrel to the floor), place the upper on the inside of one knee and the grip inside the other, and then use my legs to squeeze the upper and lower together so that the rear takedown pin locks in place. There should be no more resistance, when using this method, than before you installed the accu-wedge. Obviously, if you are using a scope, this method may not be a good idea.