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.
MSRP: $219.95 (competition model, shown), $279.95 (skeletonized).