PHP2ICQ is a libary which lets you easily send ICQ Messages from your own php scripts by using a direct connection to the ICQ server via the Oscar protocol (ICQ v7/v8/v9). It is completely written from scratch.
Internet Relay Jabber (or IRJ) is a Jabber client for IRC. It's useful if you like IRC, but your friends use another IM service. It is written in Perl and uses Net::Jabber and Net::IRC. (This project is no longer under active development)
J.Jab. Lite is a lightweight Jabber client written as a Java 1.4.1 applet. Allows users to chat with others on Jabber, AIM, ICQ, MSN, and Yahoo im systems. Has a slimmed down version written in java 1.1.7 that allows simple group chat and moderator access
voltMessenger is a web-centered IM system. It consists in a server written in PHP that can run on most webservers, and a client written in C++. Easily deployable in office environments or as a website extension. THIS PROJECT HAS BEEN SHUT DOWN.