If you play old school games, I really recommend you find and buy the Monstrous Compendiums for 2nd edition, particularly Planescape vol. 1 and Ravenloft vol. 1 & 2 (combined). There are just a ton of great monsters in each book. They concentrate on stuff like planar monsters and Halloween/horror monsters. Even if you only use a few monsters from each book, you'll be able to liven up a campaign that just uses the standard Monster Manual.
Particular favorites for me: