IPQ (Internet Quota Project) source code
On of the more interesting projects I've developed and whose source code my employer has released under the GPL, is the Internet Quota Project (IPQ). This was used by a number of schools within the University of New South Wales to provide real-time Internet usage tracking and quotas.
The core (for which the source code is available on this page) ran on each school's gateway host. It has two main functions: a very high-speed user database where each user's TCP incoming and outgoing traffic was recorded, and a real-time TCP packet analyser which updated each user's usage in the database on a packet-by-packet basis.
The core also has an administration interface — usable by humans but optimised for scripts — through which other software, such as web proxies and administration tools, can query or update the database.
This software is mostly of historical interest now. It was developed and used at a time when the university, which was and is effectively each school's Internet Server Provider, charged rather exorbitant amounts to the schools for Internet traffic. The university's charging model changed some years later and IPQ fell into irrelevance and disuse.