well, this isn't exactly the Fate OOC, but since I never created that thread, we can use this one for that as well.
As it turns out, one of the summoned servants is a class that I made up. The Known Classes are Saber, Archer, Lancer, Rider, Berserker, Caster, Assassin, Avenger, and Saver. Defender is a theoretical class that never got added. Along with the one I've created, which I've dubbed Evoker for the time being, and Joker, there are 9 official classes and 12 unofficial.
Actually, come to think of it, there was one other class I've seen. It was a secret boss in fate extra taken from Kara no kyoukai, known as Monster.