Product Hunt is a great (and free) place to start
Personal Networks, writing blog posts about what you've learned, interact with people (genuinely) on twitter (and obviously link your project in your account), Reddit is also good. With any of these respect the community and don't spam them (you don't seem the type given your post here, thought I would mention anyways)
If its technical and you know where the people are who benefit go and interact with them. Help them with something they need help with. Then get them to try it, find issues, fix 'em, rinse repeat.
It takes a lot of footwork!