The definition of on-topic has narrowed over the years, so that "what is a good library to X?" and "is there a useful tool to X?" are now off-topic. If such questions get asked, they will be closed before you can answer with a link to your tool.
The way I see it you have three, possibly four options:
Put a link to the tool in your profile. Answer tons of questions without ever mentioning your tool, using the knowledge you gained by building it. Some people will check out your profile and follow the link. Also, become an active blogger or tweeter or suchlike, and put links to those in your profile. Some people will promote your tweets and blogs for you, which will end up promoting your tool. This violates no rules of the site and will work, but it might take time and requires you to be awesome rather than just good.
Look for questions from people who are trying to do whatever it is your tool makes easier. Answer the question. If something takes 20 lines or whatever to do, or seven steps, explain that. And gently mention something like "yes, this is a lot of trouble. I made a library to do it myself. There's a link in my profile." Nothing stronger than that. AT NO POINT should you consider posting identical answers to multiple questions. Each answer should be specific to each question.
Ask questions that you think could be answered as in the second bullet, and provide good answers to them, again possibly with your parenthetical.
if you get email support requests, consider posting them as questions and then answering them, and emailing the asker a link. I used to do this in my blog: if someone emailed a question I would blog the answer. As long as the questions are real, this is ok. Eg "I am using the XYZ tool and getting this error message. What does it mean?" is a real support question. It's even possible that users who get these links from you will ask their own questions here next time. (Careful with this one, though. See http://meta.stackexchange.com/a/217355/147247 )
If you start answering for no other reason than to promote your tool, or if there's no information in your answer other than promoting your tool, expect to get spam-flagged and perhaps see your answers (or even your account) deleted. Always err on the side of discretion. Once people realize you really know your stuff, word will get out about your tool. This site is a great way to help people realize you really know your stuff. It's designed for that. It's not a great way to get the word out about your tool. You have to live with that.