One who uses or makes use of something, a consumer/client or an express or implied licensee (free user) or a trespasser.
A person who uses drugs, especially illegal drugs.
A person who uses a computer or a computing network, especially a person who has received a user account.
An exploiter, an abuser (a person who "uses" people, that is treats and regards people unfairly, selfishly and/or unethically).