Dare I say I think Monsternomicon is a bit overrated? The art is nice, and it has some neat ideas and good approach to background info, but if you aren't playing Iron Kinngdoms, you may not find it as useful as others here might suggest. Many creatures are in the steampunk vein, are specific to IK cosmology, and/or are IK specific rehashes of common MM creatures.
Two of my favored monster books for the sheer variety and freshness (or classicness) of their ideas are Tome of Horrors and Creature Collection II.
I have also found a lot to like in the smaller, more specific creature collections. One of my all time favorites is Green Ronin's Legions of Hell, in which all of the creatures are loaded with plot ideas. FFG's lore series is very nice, especially Necromantic Lore (covering undead) and Twisted Lore (covering abberations, oozes, and shapechangers).
Kenzer's new monster book looks nice, but I can't attest to it's usefulness yet.