Impetus
(n.) the force that makes something happen or happen more quickly
Catalyst
(n.) a person or thing that precipitates an event
Instigate
(v.) bring about or initiate (an action or event)
Evoke
(v.) bring or recall to the conscious mind
Incite
(v.) encourage or stir up (violent or unlawful behavior)
Inspire
(v.) fill (someone) with the urge or ability to do or feel something, especially to do something creative