I'd say a bulk of the monsters "present" in my campaign are from the MM, although there are quite a few "basics" in it: Animals to Dragons, Goblins to Fiends, etc. Of course, there are a few that I don't use or those that I have renamed/redescribed (ex: Owlbears are called Gu'Nok Bears and look more like vicious mutant bears rather than a crossbread).
Outside of that, I draw from quite a few sources. Tome of Horrors and Liber Bestarius are high on my list; I also commonly re-use monsters from Minions: Reborn, Nyambe, Necropolis, and Creature Collections 1 & 2. I also have a few flagged out from Chaositech, Creatures of Rokugan, Midnight, and Monsternomicon for future use.
[Edit: Add Creature Catalogue to the "commonly re-used" list... Thanks for the reminder, John!]
I also plan on using most of the Demons/Devils from Armies of the Abyss and Legions of Hell, although I'll likely be getting the compiled/updated book before I do, or I might just update them myself.
I've also used quite a few critters that were released in Fantasy Flight's and AEG pamphlet-sized adventures, as well as gleamed a few from other sources that had a few monsters as extra crunch (adventures, books focusing on a single magical school, etc.).