How to sell software tools to developers?

I have Windows desktop tools for developers. I have a few large companies that bought them. It is mostly project managers that did the buying and so far they found me via Google search(I have done a pretty good job with SEO). I have some good testimonials on the website now.

What are some effective ways to reach them with a marketing budget of $1000 US/ month?

If possible, you can offer licenses for Open Source projects for free. If you tool is good, people will start to use it on their daily jobs and buy a company license.

This way it's done by Atlassian, oXygen and many others. And it has only a low cost.

Second idea, you could give a few licenses to some well known technical bloggers in exchange to a review. I don't know a single dev who doesn't read blogs.

Hope that helps a bit as a starter.

Cheers
Christian