Nobody is going to simply stumble across your application, so it's your job to start promoting it! That doesn't mean paying for advertising, but marketing doesn't always cost money.

If you're keen to have more people use it, you need to be able to clearly answer:

What is the benefit of using your application? What does your tool do to make people's lives easier/better? How can people access your application?

Then you start with your own social networks. Explicitly ask people to use your tool as testers, find people who you think would benefit and approach them directly. Tell people why it's cool and ask them to share it - then go from there.