XChat for Linux is an open-source IRC (Internet Relay Chat) client for Linux. It is designed to be used for group chat, private messaging, and chatting with multiple people in different channels. It supports a wide range of features, such as DCC chat, DCC file transfer, text formatting, multiple servers, and auto-connect. XChat for Linux also has a powerful scripting language, allowing users to customize and extend the functionality of the app. XChat for Linux is easy to use, and provides a secure, reliable, and fast way to communicate with others online.
While not a replacement, ChatZilla is more lightweight and installs as a browser add-on. It is HTML5-based and will run on any platform that has a Mozzila-based browser that uses XPI extensions, such as Firefox. (This includes various mobile platforms.)
Suspended Shout development stopped in early 2016 and was replaced by its official community fork: http://alternativeapp.info/software/the-lounge/. See https://github.com/erming/shout
Based off the XChat code base. Actively updated and developed. This is an almost "drop in" replacement.