The Reaction Rod by Geissele Automatics is awesome. Not only is it a formidable weapon of opportunity, but it makes working on AR-15 / M-4 rifles a lot easier than the old aluminum upper receiver vice blocks. The way it works is by locking itself into the star chamber of your barrel. This has the benefit, especially when installing muzzle devices, to transfer all the torque to the barrel, made of steel, rather than the aluminum upper receiver. A great piece of gear that is ridiculously simple but hugely functional.
You can go here for the Geissele Automatics Reaction Rod.