Someone on the WotC message boards, back before 3e became de rigeur, wondered about a character playing a Doppleganger. The general consensus was that such a character would be way too powerful, but I instead came up with an alternate race that he could try. Thus, I want to introduce the Lesser Doppelganger.
First of all, to everyone else, they're playing a human fighter or thief, but they must earn double the usual experience points. This means that their movement rate is 12, their AC is 10, and they make saves as a member of their class, not a 10th level fighter.
Secondly, they do have shape-changing powers. However, they can only change into shapes they are proficient in, and are limited to humanoids between 4 and 8 feet tall. How do they become proficient in shapes? They start out with 2 slots to spend on shapes (each shape costs only one slot), and get a free one every three levels. They can spend more proficiency slots (weapon or non-weapon) on shapes if they like, but that limits their options in other areas. Slots can be kept open so they can be used when needed, but once filled, they're filled for good. They have a 90% chance to imitate the voice of any shape they are proficient in... 100% if it is simply a person they made up. A bonus side-effect is that they're largely immune to any spell that would change their shape. If such a spell or effect affects them while not in their natural form, they simply have to switch to another form, then switch back. If it affects their natural form, then that alteration is permanent, though they can still turn into their other forms without a problem.
For example, Meta the Lesser Doppelganger is hit with a Polymorph Other spell while in his Ogre form, turning him into a dung-beetle. Not liking this form, he turns into his alter-ego, Skippy the Halfling, and then back into Smash Ogre (while the wizard who polymorphed him is doing all sorts of other nasty things to him). Later, while searching the now-dead wizard in his natural form, Meta finds an interesting girdle on the body. Being somewhat impetuous, Meta puts it on, only to learn that it's a girdle of femininity/masculinity. From now on, Meta's natural form is that of a woman. However, he can freely turn into Skippy the Halfling and still look exactly the same.
Shapechanging has some limitations, though:
1) Changing shape requires a proficiency check, and a full round. I suggest either Wisdom or Charisma - 1 or 2. Turning back to their natural, human-like form, though, is free (though it still takes a round to change).
2) They can only turn into shapes that have a base hit die equal to or less than their level, and are corporeal in nature (no looking like a wraith or a banshee to scare people). Thus, if they want, they can look just like Elminster at 1st level, since they both are "human" and have the same base HD. They cannot, however, look like an ogre until 4th level, because it is a 4 HD creature.
3) They do not get any special, non-physical abilities from their new form. They can look like Elminster, but they can't cast spells, don't know what he knows, and don't necessarily know how to act like him. The disguise proficiency is a must for these characters, so they can at least get mannerisms down. However, if they turn into a Minotaur, they have great senses, and head butt and bite attacks. They don't get any bonus ability scores, however, so that Minotaur form is less believable if they can't back it up with a 18 or better strength. They also don't automatically speak the language, so that's another barrier in a good shape-change.
4) Only their bodies change. This means that, when going from their natural form, they might destroy any clothing they have on, and might hurt themselves with their armor. I would suggest for those who try to burst out of their armor you inflict one point of damage for every non-magical AC point the armor had, and ruin the armor.
5) Prejudice. Even on highly magical worlds, people aren't used to seeing someone just switching shapes at will. Also, if there is one of these people acting as a hero, you can bet there were hundreds of them over the centuries acting as villains, so people are not going to like these Lesser Doppelgangers. This means the character will have to be careful about his shape-changing, or lose his head.
Once they've changed shape, they can hold that shape until they're knocked unconscious, die, or decide to change shapes again. If they go to sleep naturally, they have to rest for 1 round per hour of sleep first, then make a proficiency check in order to keep the shape for the night. If they're forced to sleep unnaturally (_Sleep_ spell or similar effect), they automatically lose the shape.