While some might view this as hyperbole, but I'm going to say it anyway: Green Ronin's Advanced Bestiary was easily one of the best books put out for 3rd edition Dungeons & Dragons. Its a book I believe anyone who enjoys the d20 system should have on their shelves because it gives you a number of interesting templates to add to your typical monsters and make them different and unique (who wouldn't want to fight clockwork chokers?).
Today, Chris Pramas announced on the Paizo forums that Green Ronin is running a kickstarter to update the Advanced Bestiary to the Pathfinder RPG. Since Green Ronin usually publishes very high-quality products and this was one of the best books published under the OGL, I definitely feel comfortable promoting this kickstarter.
If you play or run Pathfinder, like Green Ronin's work, and want a book that will give you a lot of material to make some really cool and interesting encounters, I highly recommend you back the project. If you're interested, click here.