One of the AR-15 platform's greatest strengths is the amount of customization that can be put into the rifle by the end user. There's a whole world of accessories that don't require a lifetime of knowledge to install, and there are many ways to make one specific rifle your very own.
There are many tools available for those who wish to do their own tinkering. Brownells carries a good number of them, and a few of the most popular are the blocks used to stabilize either the upper or the lower receiver of the AR-15. Should you wish to install an optic, iron sights or a new handguard, the upper receiver action block protects the upper's finish and also provides support internally with a bolt-shaped block.
Should you wish to complete a stripped lower or simply install a new buttstock, the lower receiver vise block fits into the magazine well so that it, not your lower, can be firmly secured in a vise for all your installation and customization needs.
There are hundreds of additional blocks, pins, presses and other useful tools available to help you make a particular rifle your own.