LarryMessenger is a simple to install and easy to use cross-platform instant messenger client. It has a nice graphical user interface, and allows people to communicate over a LAN with static IP addresses without the need for a central server. Furthermore, it provides functions to query the ICQ and AIM networks.
Rxpd plugin for XChat is a plugin for the XChat IRC client which lets users filter the incoming conversation with the RegexPolicyDaemon. This is used to ignore spam, trolls, and other abusive content (at the user's option). People can publish their ignore rules and then filter or merge published rules from friends into their own rules, thus providing a distributed peer to peer system.