from the OED:
freak (fri:k), sb. [Not found before 16th c.; possibly introduced from dialects, and cognate with OE. frician (Matt. xi. 17) to dance.]
1. A sudden causeless change or turn of mind; a capricious humour, notion, whim, or vagary.
2. The disposition of a mind subject to such humours; capriciousness.
3. A capricious prank or trick, a caper.
4.a. A product of irregular or sportive fancy.
4.b. (More fully freak of nature, = lusus naturae): A monstrosity, an abnormally developed individual of any species; in recent use (esp U.S.), a living curiosity exhibited in a show.
4.c. One who 'freaks out'; a drug addict.
4.d. With qualifying word or phrase: one who shows great enthusiasm for the activity, person, or thing specified . . . an aficionado.
5. [T]o denote something abnormal or capriciously irregular; freak show, at a fair, etc: a sideshow featuring freaks.
from The Random House Historical Dictionary of American Slang (vol 1.)
1. a person who is markedly or offensively eccentric in dress or behavior; Weirdo.
2. an ardent or extreme devotee, practitioner, or enthusiast.--usu. in comb. with prec. noun.
3.a. [A] person, esp. a homosexual or a prostitute's customer, who habitually engages in unorthodox sexual practices; (usu. in comb.) a sexual fetishist.--often used with neutral force.
3.b. [A] sexually passionate or sexy young woman; (specif.) a nymphomaniac.
4. a beatnik or hippie. [The usual designation among hippies, used with neutral or positive force]
5. Narc. a person addicted to the use of a drug.
Other interesting variants:
freakazoid. n., 1. Weirdo., freaking adj. & adv. (euphem. for) Fucking; Frigging. freak [off] v. 1. to masturbate; (fig) to obtain sexual gratification. freak [out] v. 1. to induce usu. overpowering astonishment, anger, fear, or confusion in; cause to lose composure, sense, or sanity.