syslog-ng is an enhanced log daemon, supporting a wide range of input and output methods: syslog, unstructured text, queueing, SQL & NoSQL.
Documentation sources for syslog-ng Open Source Edition (https://github.com/balabit/syslog-ng)
qlogsystem is a very efficient and easy to use logger library written in C++ (using the Qt framework).
IPTables addons relate to Zorp (sevice, zone match) and KZorp (kzorp target).
eventlog is a library for creating structured events from applications, and is the workhorse behind syslog-ng's own messages.