php Free Chat is a simple, fast, and customizable chat server that uses the filesystem for message and nickname storage. It uses AJAX to smoothly refresh and display the chat zone and the nickname zone. It supports customized CSS stylesheets and a plugin system that allows you to write your own storage routines.
FleaIM is a Web-based MSN messenger clone. It can be used to register, sign in, automatically login. You can use it to modify your profile, state, and signature. It can send and receive messages, flash, and face icons. It can search, add, block, remove, and sort contacts. You can use it to view and download chatting history. It supports sound notifications for incoming messages, contact logging, etc. It can also maintain contact groups.