I've used several of these sources at one time or another, but the two big ones that get used all the time are the Monsternomicon and the Book of Fiends.
Aside from the MM, I sometimes use all the WotC books in the poll, ToH I & II, Monsters of Faerûn and other FR books, and the Creature Catalog hosted here at EN World.
I voted "Other". They basically come from starting with a non-standard miniature (in many cases, a plastic toy belonging to my children not originally made for D&D) and then figuring out what it is and can do based upon standard monsters that may be somewhat similar.
Heh, not counting PDFs my list is Monster Manual, Monsternomicon. I use as many Monster Geographicas as the others combined!
I do have other monster books, but I tie critter books to their settings, so if I ever run Oriental Adventures Monsternomicon will go on the shelf and Creatures of Rokugan will come out to play. MG is a good general purpose critter book that fills my needs perfectly.
Monster Manual II
Fiend Folio
Monster Manual III
Creature Collection
Creature Collection II
Creature Collection III
Homebrewed
Tome of Horrors
Tome of Horrors II
Oriental Adventures
Monsters of Faerûn
Miniatures Handbook
Eberron CS
Sharn: CoT
Magic of Faerûn
Planar Handbook
Manual of the Planes
Deities & Demigods
Psionics Handbook
FRCS
Book of Vile Darkness
Complete Book of Eldritch Might
Book of Fiends
Strange Lands: Lost Tribes of the Scarred Lands
Scott Greene's Creature Catalog
Things found on the Internet
...
OK, average monster used by source is probably between one and three... Certainly no more than 5! But still!
Wow *sniff* so much love, makes me a happy ENWorld-aholic
Surprisingly, I haven't used any of my conversions yet. I got done converting all those fiends right after I finished a fiend-focused story arc in my last campaign. Oh well, I got plenty of stuff for later campaigns...
I frequently use:
- Monster Manual II
- Fiend Folio
- Monster Manual III
- Draconomicon
In my Eberron Campaign, I'm planning on making heavy use of some Hooded Pupils (Libris Mortis), Half-Vampires (Libris Mortis), Bonedrinkers (MMIII), and Boneclaws (MMIII) in the upcoming games.
Later on, I'll be doing some cold-focused quests (Frostburn, MMII, MMIII will all be used!).