A reasonably reliable traffic per application monitor using BPF.
Receive notifications whenever a new program connects to the network, or when it's modified. Monitors your bandwidth, breaking down traffic by executable, hash, parent, domain, port, or user over time. Uses BPF for accurate, low overhead bandwidth monitoring and fanotify to watch executables for modification.

A new implementation of STUN and TURN. Full IPv6 support. Supports server authentication with the REST APi by the RFC. Implemented with Erlang.

An RPC server written in Python designed for controlling the firewall rules of a captive portal system.

Oskar Andreasson's Ultimate IPtables Tutorial for Linux. Considered one of the best ways to learn how to set up and use IP tables. Comes in multiple formats and several languages, so there should be something for everybody there.

A website that tests to see if arbitrary URLs are accessible from within China (behind Golden Shield/The Great Firewall).