All the normal classes from the PHB are available. I am removing multiclassing restrictions from the monk and paladin classes.
Scout and ninja from the complete adventurer are available as written.
Swashbuckler and hexblade from the complete warrior are available, but will need to be modified prior to use. Both classes as written are overly powerful if you take just a few levels of them, but are not viable options for straight classing. Hexblades are fairly rare in the setting, and usually (though not always) function as an evil equivalent to paladins.
There are two spontaneous casting divine classes in the setting: path of dreams, and path of nightmares. Both of these are statted well enough to be usable for this game, but are extremely rare in the setting; if you want more information, ask.
Warlocks are available, and the class has been heavily modified for balance. Warlocks are very rare in the setting, and will usually register to detect evil spells, even if they aren't.
Prestige classes are all with GM permission only, though permission is likely to be granted.
Due to the freeform nature of the game, character creation may be fudged a little - you can, for example, stat a ranger who trades in favored enemy and animal companion abilities for increased spellcasting.
I am changing some base classes to make straight classing a more viable option.
For example, Sorcerer gets 4+ skill points and has diplomacy, gather information, disguise, and use magic device as in class skills. They also gain a spell-like ability usable once per day at level seven (and a new one every four levels thereafter). This ability may be chosen from any spell available to a wizard or bard of their sorcerer level, a druid of their sorcerer level minus two, or cleric of their level minus four. This is explicitly based on levels of the sorcerer base class - prestige class levels do not grant further spell like abilities, nor allow higher level abilities to be chosen if more levels of sorcerer are taken at a later time. Caster level of these abilities is equal to the total sorcerer caster level of the character, including any prestige classes, however.
Other classes that will get bonuses of some sort from this set of adjustments: fighter, wizard, paladin. Note that for these, there will be no meaningful adjustments prior to about level seven.
Class: Warrior. Five level base class. Does not count towards calculating multiclassing xp penalties. D8 hit die, good fort save, fighter base attack, 5+ skills. Gains proficiency with all simple weapons, light armor, and shields. Gains one martial weapon proficiency at first level, and a second martial weapon proficiency at second level. Gains medium armor proficiency at third level. Gains one fighter bonus feat at fourth level. Gains skill mastery with one skill (must be an in class skill) at third level. Gains Master of Irrelevant Studies at fifth level - the skill you selected at third level is now an in class skill for all further levels you take. Class skills: Fighter list, plus: profession, spot, listen, sense motive, heal, survival, and two skills of choice.
Paladins: The goal of the order of paladins is to prevent the destruction of the world. There are two primary focuses of this effort: opposing demons - who specifically are trying to bring about the destruction of the world; and destroying undead and those who channel negative energy - as their mere presence disrupts the overall life energy of the world. Loss of paladin powers is only triggered by failing to do something about known demon or undead problems (reporting them to another, presumably more capable, paladin is doing something), completely and consistently failing to do anything that might bring such problems to your attention (typically requiring years of isolation and inaction), or, of course, the obvious of knowingly assisting demons, undead, unholy clerics, and so on.
A paladin may attempt to redeem foes (even demons can be affected by the atonement spell), but the order has found that simply smiting them typically proves more effective.
A paladin is also expected (though not required) to stay on the positive side of the local law. It is difficult to work effectively when there are parties of adventurers out to kill you.