The Daily Shooter has reviewed the pre-assembled drop-in trigger for AR-15 lower receivers. He reviews the single-stage 3.5 lbs version, it is also available in 4.5, 5.5 and 6.5 lbs version with either a curved trigger blade or a straight targer-style trigger. The recommended retail price for these is $196, but you get buy them for less than $170 if you do some Googleing.
I have been considering getting one of these triggers. Either this or the Timney AR-15 Competition trigger.