OK... first off, I have no vested interest in this company whatsoever, and am just posting this because it sure seems like a neat idea at a reasonable price.
Manmade rocks are no new thing, and I'm sure others have probably made something like this in the past. But there's a new company out there that's making lightweight, fake rocks that are totally flat on the back, and have magnets embedded in them.
Deep Water Rocks
They make plain ol' man made rock, but the product I'm talking about is their DWG-MAG Rocks. I ordered a piece at $30, and surprised at how un-man-made the thing looked...
I was also surprised at how light it was. The thing actually floats! Obviously the filler is plastic or styrofoam. Sure made placing it in the tank nice - didn't have to worry about dropping it and cracking the bottom! If my grip slipped, it just bobbed to the surface!
Since I'm not one to take people's word that the thing won't leach out bad stuff in my tank, I submerged the "rock" in my saltwater storage tub for about 3 weeks. Each week I tested for nitrates and phosphates. Nothing. Granted... that's not really a scientific study, but at least it didn't obviously pollute my fresh water.
So into the tank it went...
It might not look like it in the pictures, but there are TONS of little shelfs and nooks and crannies to stuff corals on this thing. (Much to the consternation of my wife! She just shook her head when she saw this show up in the mail!) In my little 46g, I definitely just added some serious real estate to my back glass!
My only complaint (minor) about the product is that I'd like the magnets to be stronger. They're definitely strong enough to hold the thing in place on the back wall, and it takes a fair amount of force to push the "rock" down the glass. But it'd just make me feel better if they had a little more power to them. But then again, if it detaches, it's going to float to the surface anyway.
I'll update the thread as it gets covered in coralline and doesn't stand out like a sore thumb anymore.