Go to forums and communities and talk directly to your buyers. A good example for that is Netflix: To spread the word, it used internet bulletin boards and forums frequented by avid DVD buyers. A good example you is Netflix. It depends on what product you try to sell. But go where your potential buyers are and talk to them.

Addressing target audiences through sites such as Reddit can work very good.

Tools:
https://canny.io/ – Great tool to turn feedback into action to drive growth and customer loyalty