Yes, one is a Scrabble word!
...and is worth 3 points in Scrabble, and 4 points in Words with Friends
one
Adjective
1. Used of a single unit or thing; not two or more
2. Having the indivisible character of a unit
3. Of the same kind or quality
4. Used informally as an intensifier
5. Indefinite in time or position
6. Being a single entity made by combining separate components
7. Eminent beyond or above comparison
Noun
1. The smallest whole number or a numeral representing this number
2. A single person or thing
What Scrabble words can I make with the letters in one?
3 Letter Words
2 Letter Words