Almost all classes have multi-classed so it is hard to say exactly. A lot of people like to multi-class with rogue for all the benefits, such as sneak attack and evasion. Rangers are used to multi-class for Arcane Archers and ambidexterity. Fighter class for all the feats and then to Weapon Master. They are a dime a dozen. Fighting clerics and bards are fairly common. Underused - Paladins Overused - Wizards, Sorcerers and Weapon Masters. I recommend you view the server status when there are a lot of people logged on (about 20-30 people) to get a good idea of what is going on.
Weapon Masters are a dime a dozen? Really? Seems like I'm the only one on when I am on. Maybe 1 or 2 more...sorry twidget. Just not agreeing with you there. :\\
Great another person to harass and avoid. D: Daniella's brood of lackeys is growing!