Volunteer

One who enters into, or offers for, any service of his/her own free will, especially when done without pay.

One who enters into military service voluntarily, but who, when in service, is subject to discipline and regulations like other soldiers; -- opposed to conscript; specifically, a voluntary member of the organized militia of a country as distinguished from the standing army.

A person who acts out of his own will without a legal obligation, such as a donor.

A plant that grows spontaneously, without being cultivated on purpose; see volunteer plant in Wikipedia.

To enlist oneself as a volunteer.

To do or offer to do something voluntarily.

"to volunteer for doing the dishes"

To offer, usually unprompted.

"to volunteer an explanation"

To grow without human sowing or intentional cultivation.