Creature Catalog is a huge collection of older edition creatures converted to 3e/3.5e maintained right here at ENWorld. The "New Monsters" section contains beasties that have never been officially converted, while the "Creature Crypt" contains unofficial conversions for creatures that appear in 3e elsewhere (for example, things that were converted before Monster Manual 2+)

If I understand correctly, the Tome(s) of Horrors was more or less spawned from this project.

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