Do you know of any software that uses an interactive tree UI to enable users to browse discussion threads?

In commonly used posting platforms, discussions are organized in threads, and the mathematical structure of a thread is that of a tree) where each reply is a node and each edge links a post to its reply. Do you know of existing software that use this tree structure as their primary user interface for thread browsing? I am thinking of a UI that would look like this, but for discussion threads browsing.