A common complaint about the standard AR-15 parts kit is the trigger included is rudimentary, at best. Oh, sure, it will release the hammer and do the job intended, but it's hardly a precision device and is more suitable for minute-of-bad guy than minute-of-angle.
Timney Triggers has the solution in the company's excellent AR-15 trigger line. Available in 3-, 4- and 4.5-pound pull weights, these drop-in replacements swap out easily for the existing trigger and hammer in your AR-15-platform rifle. Models are available for both AR-15 and AR-10 variants, and competition and skeletonized versions are offered.
It's hard to describe what a difference a really good trigger can make until you actually experience one. I've got a Savage Model 10 with a Timney trigger installed and it is difficult to adequately catalog how much better the trigger is (my Savage pre-dates the Accu-Trigger) over the standard trigger (which was already pretty good). Upgrading the trigger in your rifle is one of the most important hardware improvements you can make for your shooting accuracy.