This site advertises itself as a "complete" listing of the various monsters from pretty much ALL the 2nd Edition AD&D settings/game worlds. I was quite surprised at the list because some of these settings I had never heard of. I'm willing to bet that you, and more importantly, your players, haven't heard of all these settings, much less the monsters from the settings:
- AD&D 2nd Editions (Generic Monsters)
- Dark Sun
- Forgotton Realms
- The Horde
- Masque of the Red Death
- Red Steel
With so many folks playing AD&D, HackMaster 4th Edition, or the various Old School Rules (OSR) games, being able to drag out a few new monsters that are already statted (or require little work to convert) is a huge boon.
If you don't play one of these compatible games you might be lucky to find an easy conversion from the 2nd Edition AD&D ruleset. Even if you have to do a lot of conversion work simply having a large store of monsters to start with might prove useful. The websites' page on monster stats might prove quite useful in making conversions.