SEOnaut is free and open-source software that can be downloaded, used, and modified by anyone under the MIT license. It is a web-based tool, which means it needs to be set up differently than a typical desktop application. If you don't want to set it up yourself, you can still use the hosted version on SEOnaut.org for free.
Although the SEOnaut software is totally free and is available under an open source license, running servers still costs money. That's why the hosted version of SEOnaut comes with some limitations.