GnuCash allows you to track bank accounts, stocks, income, and expenses. As quick and intuitive to use as a checkbook register, it is based on professional accounting principles to ensure balanced books and accurate reports. It is backed by an active development community and is blossoming into a full-fledged accounting system.
ISPacct (formerly Brickacct) accounts the logfiles which Bintec routers spew out depending upon configurable rules. Logfile-analyzing mail accounting functionality is also included (in beta stage). It has multiple backends like text, HTML, and MySQL. It was designed and used by a small ISP and is geared towards such uses as monthly accounting per IP or per dial-in use.
SAGU, which is a portuguese acronym for "Sistema Aberto de Gestão Unificada", was originaly designed to automate all of what that happens between a student and their Educational Institution. This relationship happens at the academic, financial and administrative levels, and SAGU has subsystems dedicated to these levels. SAGU can be tailored to fit the needs of any Educational Institution.
GNU Radius is a RADIUS server and an accompanying set of monitoring utilities. It features MySQL and PostgreSQL interfaces for authentication and/or logging (the ODBC interface allows you to use almost any existing DBMS for that purpose), the ability to rewrite RADIUS requests from various NASs to normalize them to a more understandable format, and the ability to completely customize the behavior of RADIUS authentication and accounting based on NAS and user attributes.