What monster books have you found useful or are buying?

[Voadam]

Denizens of Darkness (the ravenloft monster book) has tons of templates and low to mid level horror critters.

Diablo II Diablerie has the 3e monsters, it is a poor update of the 2e product, it does not include succubus, the advisor demons etc. and the balrogs went from doing 15d6 a round to a pathetic couple of d4s a round with their breath blasts. Overall Diablerie is a poor product but it has some OK monsters in it.

 

[Desdichado]

I own, enjoy, and have used all of the following:

• Monster Manual
• Creature Collection
• Creature Collection II
• Creatures of Rokugan
• Legions of Hell
• Armies of the Abyss
• Monsters of Faerun
• Alien Anthologies (alright, it's Star Wars, but I've stolen pictures, at least, occasionally)

I also own the following which, while not technically monster books, have tons of monsters, some of which I find indespensible:

• Manual of the Planes
• Epic Level Handbook
• Oriental Adventures
• FRCS

And finally, my plans on monster books are to pick up the following announced products, although I haven't had a chance to yet:

• Monsternomicon
• Liber Beastarius
• Monster Manual II
• Fiend Folio
• Denizens of Darkness
• Possibly also Tome of Horrors -- although I have the pdf from back when that was available, so I'm not sure how much I need it.

 

[Nightfall]

Everything WotC is worth it. Haven't looked much into monster books by other publishers, but the reviews section will help you on that. I can tell you that Mystic Eye Games' Nightmares & Dreams books are good, but that's about it.
However... I'm SO getting Tome of Horrors!

And yet you've not fought any of the monsters I'm planning on throwing at you? Interesting...

And Warp's right. They are going to fix up CC1 in a pretty cool way. A few more templates (like the Unhallowed for starters) as well as ELCs on some of the humaniod monsters (like Harpies and Mantacori.)