There are lots of ways to advertise that don't resemble cold calling, spam, or banner ads. Probably the simplest is article writing.

Write about a problem your customers have, and explain how your product can solve it. Write about a subject popular with your target audience and bring your product into the story (or just stick in a promo paragraph at the end).

If you write well and have a good product, people will find you, and they will tell their friends.