foxTalkz is the total solution for chatting. It is built as a Firefox extension integrated with Google chat, and it does not need an internet connection to chat in the LAN.
Y-Chat is a Chat program written in Java and html, accessible with a simple web browser through the internet (or even in LAN if wished). This only requires a JDK on the hosting server and a web browser for the end-user.
LANCon is a LANParty and LAN Tournament management system which helps organize and run a LAN Party or Gaming event. LANCon is written in ASP (VBScript) and doesn't require a Database since all data is stored in XML.