Yes, PEG is a Scrabble word!

Letter values in Scrabble US: P 3, E 1, G 2.

Is it a valid word?

PEG is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

a wooden pin pushed or driven into a surface
small markers inserted into a surface to mark scores or define locations etc.
informal terms for the leg
a prosthesis that replaces a missing leg
regulator that can be turned to regulate the pitch of the strings of a stringed instrument
a holder attached to the gunwale of a boat that holds the oar in place and acts as a fulcrum for rowing

succeed in obtaining a position
pierce with a wooden pin or knock or thrust a wooden pin into
fasten or secure with a wooden pin
stabilize (the price of a commodity or an exchange rate) by legislation or market operations

Also: wooden leg, nail down, peg down.

Not playable in Scrabble US: PEGLEG.