General:
* Motivation
* Stimulation
* Encouragement
* Elicitation
* Impetus
* Incitement
* Arousal
* Excitement
* Influence
* Idea
More specific:
* Muse (referring to a source of artistic inspiration)
* Ethereal (referring to a feeling of inspiration that is otherworldly)
* Illumination (referring to a sudden moment of understanding or inspiration)
* Enlightenment (referring to a deeper understanding or wisdom gained through inspiration)
The best synonym for "inspiration" will depend on the specific context and the nuance you are trying to convey.