Monster Compendium: Monsters of Faer¿n (A D&D(r) Accessory) 
Demons and half-demons, dragons and dragonkin, animated corpses and restless spirits, wielders of magic and eaters of spells: These are the creatures of Faerûn, the monsters of the Forgotten Realms campaign setting.
Monster Compendium: Monsters of Faerûn contains scores of new monsters for use in Dungeons & Dragons adventures. From the aarakocra to the Tyrantfog zombie, these monsters present a whole new range of challenges.
Although usable in any campaign, these monsters are especially suited for the Forgotten Realms setting -- a world of great magic, terrible villains, and high adventure.
Reviews
Though it only provides a fraction of the monsters given in the Monster Manual, the Monster Compendium still provides a lot of info and even some extra realms-specific info on location and habits. The monsters are interesting, and the stories behind them are also quite intriguing. And some of the monsters are just downright awesome (Fang Dragon, anyone?)
However, to be honest, the small amount of monsters featured is a disappointment. Still, the book as a whole is a very good one, and a necessity for any 3e FR campaign.
Basically the moral of the story is if you're going to play 3rd edition, be sure you fully check over the new descriptions of the monsters. They may have totally different powers, or they may be more or less powerful. Something you thought was foolproof against certain monsters now may do nothing.