The AR-15 is a great rifle.  It's reliable, accurate, and made in America.  What else could you ask for?  Unfortunately the triggers in many models don't do the rest of the weapon justice.  A rough action, heavy trigger pull, and lots of slack. There are literally dozens of aftermarket triggers available.  They range in price from $58 to nearly $300.  If you aren't an experienced AR-15 shooter, a $200+ Geissele trigger probably won't change your life.  However, a cheaper alternative will make a world of difference over a cheap stock trigger and quickly improve your accuracy.
The stock trigger on my Smith & Wesson MP-15 was terrible in relation to the after market triggers.  For $65 a Spike's Tactical Battle Trigger is well worth the money.  It is not a match trigger, but the pull is much lighter, has a short reset, and very little slack before breaking.  It's is also coated in Nickel Boron.
The installation is pretty easy, and only takes about 15 minutes.  I recommend taking the trigger assembly out and cleaning it every few thousands rounds to maintain a crisp trigger break.