I wondered the same thing about a week ago. I ordered the Scythe Ninja. I was afraid that it would be heavy, but to be honest, it's a lot lighter than I thought! It also works great and is extremely quiet! But prepare to pay the price. I was going to get the Arctic Cooler Freezer 7 Pro or whatever, but after being reassured of not having to worry about the mobo snapping, I just snagged something that was bigger, but not that heavy. I also like it because it doesn't require the back plate... at least not the Rev B. However, you might have to remove the motherboard to get it to sit in. The cracks in the corners are there to allow you to use a long flat head screw driver to lock them in. Make note though, you actually turn them the opposite way that the arrow says! *shrugs* perhaps I got some bad pins, but everything works great! Overclocked and my CPU never gets past 55C. That's with a 25% overclock on a E6600 without any voltage changes and in a P180B case. Heatsinks that you think might strain your mobo, come with back mounting plates.
NOTE: DO NOT USE TAPE IF YOU PLAN ON REMOVING IT LATER! IT'S A PAIN!!! =X I had to do that earlier this week 🙁
My $.02