here are some from my own collection that I find are must haves:
Legends and Lairs: Darkness and Dread. I love the cut down classes, outside of E6, it is the only other setup that can do a low powered game (that I have seen). the only issue I have is the "entry requirements" for some of the classes, so where it says level + 3 on a prerequisite skill, im houserulling it to just a straight up 3.
Uneartheed Arcana. Optional rules galore. I initially ignored this becasue of all the racial variants, but the rules for wounds & vitality, action points, traits, and other things will now be a part of my games.
City State of the Invincible Overlord. I havent had a chance to actually run through this, but there is so much informtaion here, and it all seems so cohesive and well thought out.