Mailing the story:

OpenSnitch, an application firewall for Linux, is no longer under active development. This didn't mean it was the end for this project though, because Gustavo forked it about 8 months ago, and has been constantly improving it since.

The application monitors outbound connections that your applications are trying to make, preventing or permitting their connection based on a set of rules (the user is prompted to allow or deny access when no existing rules are found).