The classes in the PHB are all available for characters, though I don't recommend taking Druid. If someone REALLY wants to play one, I'll make it work, but if none of the PCs are Druids, it's entirely possible that Druids will simply not exist (at least not in the known world).
If you're interested in playing an alternate base class, such as the ones that appear in the Complete Warrior/Divine/Arcane/Adventurer series, I'll take that into consideration, but I make no guarantees. Anything not in the three core rulebooks is by DM approval, only (but I'm usually pretty lenient)!