Almost anything. Beholders are one of the creatures specifically not present. But there are...
Angel, Astral Deva
Aranea
Archon, Hound
Archon, Trumpet
Athach
Barghest
Belker
Centaur
Demon, Succubus
Demon, Vrock
Devil, Hamatula
Devil, Imp
Devil, Kyton
Drider
Eladrin, Ghaele
Elemental, Air
Elemental, Earth
Elemental, Fire
Elemental, Water
Ettercap
Genie, Djinni
Genie, Efreeti
Genie, Janni
Ghoul
Giant, Fire
Giant, Frost
Giant, Stone
Golem, Flesh
Griffon
Guardinal, Avoral
Hag, Annis
Hag, Green
Hag, Sea
Harpy
Lillend
Magmin
Medusa
Mind Flayer
Minotaur
Mummy
Ogre
Ogre Mage
Rakshasa
Salamander, Flamebrother
Satyr
Shadow
Sprite, Grig
Sprite, Nixie
Sprite, Pixie
Treant
Triton
Troll
Umber Hulk
In addition, there was a series of articles on the WotC website by Sean K. Reynolds that introduced the concept of Template Classes, which applied similar rules to various templates.
I've done revisions of most of the monster classes listed above and have done up several other monster classes from other sources such as Monster Manual II.