I have my Remington 870 & Benelli M4 tactical shotguns equipped with Burris FastFire III red dot sights. They're incredibly small and lightweight and they're still working perfectly after withstanding the recoil of, in the case of the FastFire mounted on my M4, over a thousand shotgun shells. The one on my 870 has several hundred firings under its belt so far. I'm so impressed with their performance that I just bought a third one for my other Remington.
Here's a pic of my M4 with its FastFire III.
P.S.- I think the next red dot I buy is going to be what Don above has. Not that I have any complaints with the FastFires, but you can't beat the battery life of the Aimpoint optics.