HtmlUnit
HtmlUnit is a "GUI-Less browser for Java programs".
It models HTML documents and provides an API that allows you to invoke pages, fill out forms, click links, etc... just like you do in your "normal" browser.
It has fairly good JavaScript support (which is constantly improving) and is able to work even with quite complex AJAX libraries, simulating Chrome, Firefox or Internet Explorer depending on the configuration used.
It is typically used for testing purposes or to retrieve information from web sites.
I made my first contributions to the project in 2008; I'm an official committer since 2010 and being the (only) maintainer since 2017.
The source code is available at GitHub and regular update information are available on Twitter.