Paladin (Cleric Variant). The Paladin is like a Cleric in every respect except as follows. In weaponry they are limited to swords (one or two-handed), daggers, and lances. They turn undead as a cleric one level lower (i.e., at first level they cannot turn at all), but they are entitled to one more first-level Cleric spell per day than normal Clerics, which they receive at first level. The Paladin must be Lawful in alignment.
Druid (Cleric Variant). The Druid is like a cleric in every respect except as follows. They are limited to leather armor, may only use clubs, staves, or daggers, and instead of turning undead, they "calm animals" using the turn undead table ("turned" results mean the animals flee, "destroyed" results mean the animals become calm or even submissive). Additionally, Druids are entitled to one more first-level Cleric spell per day than normal Clerics, which they receive at first level. The Druid must be Neutral in alignment.
Illusionist (Magic-User Variant). The Illusionist is like a Magic-User in every respect except that certain spells are treated as being one spell-level lower while others are treated as being one level higher. Spells treated as one level lower are: Phantasmal Force (1), Charm Monster (3), Confusion (3), Hallucinatory Terrain (3), Feeblemind (4), Project Image (5). Spells treated one level higher are: Magic Missile* (2), Sleep (2), Web (3), Fireball (4), Hold Person (4), Lightning Bolt (4), Cloudkill (6), Hold Monster (6).
Ranger (Thief Variant). The Ranger is like a Thief in every respect except as follows. Rangers are limited to bows (long or short), one-handed swords, one-handed axes, and daggers. The Thievery ability is replaced by "Woodcraft," enabling the Ranger to move silently, hide in shadows, hunt and trap, forage, track, and identify plants and animals. Rangers cannot back stab, but are surprised less frequently than other characters, and they get +1 when binding wounds (self or others). Only humans can be Rangers. The Ranger must be Neutral in alignment.
Some may be surprised that my Ranger and Paladin are not based on the Fighter class, but on the Thief and Cleric. This is because I wanted to ensure that no class could match the straight Fighter in combat bonuses, hit points or weapon (and armor) variety.
*I have Magic Missile in my White Box game as a house rule.