IRC

Very customizable, ZNC integration, iCloud sync ($4.99). (source) macOS

One window for multiple servers, keyboard shortcuts, fast & stable. (source) macOS iOS

Based on XChat, easy to use, spell check & multiple languages. (source) Windows macOS Linux

Uses the chrome.sockets APIs to connect directly to IRC servers without needing a proxy. (source) Chrome

Distributed (clients can attach to and detach from a central core that stays permanently online. (source) Linux macOS Windows

For use in Emacs, sane defaults. Emacs

User-friendly, based on GNOME / GTK+. (source) Linux Windows macOS

Free, portable, based on Qt GUI toolkit. (source) Linux macOS Windows

User-friendly client built on the KDE Platform. (source) Linux

Feature-full, actively maintained Android IRC client. Android

Friendly IRC client built in Vala and GTK, designed for elementary OS. (source) Linux

Modern WebSocket IRC client with Discord-like UI. (source) Linux Windows macOS Android iOS Web

Powerful modern IRC messenger for the web. (source, demo) Web

Never has a client offered such granular settings for every aspect of the IRC experience. (features, screenshots) Windows WINE

Server will allow clients to connect, join channels, change topics; basic stuff.

Modular, stable, written from scratch. C++

Very simple and limited.

Portable and lightweight for small or private networks. C

Modern server that's portable and designed around specifications (bleeding-edge IRCv3 support). Go

Modular, advanced IRCd serving thousands of networks since 1999. C

IRC server without netsplits. Go

Designed for large networks with high scalability requirements. C

Designed for flexibility and ease of use. C++

Oldest IRC bot still in active development. Feature rich, uses Tcl scripting. (C)

Tonnes of ready made features, tutorial, fully documented. (Python)

Robust, user friendly, developer friendly. Python

Takes input from the chat and presses the corresponding key. Python

Main goals are simplicity and power. Python

User-friendly and powerful. Clojure

Compiles and runs C++ code snippets. C++

Simple, fast, expandable. Python

Large default plugin set, Cinch-based. Ruby

Pluggable chat bot supporting multiple protocols. Python

Extendable chat/channel/server-managenent service-providing bot. Using the Cinch Framework (Ruby)

Modular, event-driven bot featuring a REST API, individual user settings and much more. (bitbot.dev) Python

Python Twisted IRC bot with a focus on ease of plugin development. Python

Python Twisted IRC bot with a focus on ease of plugin development. Python

An archiving tool with an IRC interface integrated with various archiving services.

New generation LLM-powered bot with lua scripting support. Go

Off-the-Record (OTR) messaging plugin for irssi. C

Off-the-Record (OTR) messaging plugin for WeeChat. Python

Blowfish encryption in ECB/CBC modes with Diffie-Hellman key exchange for irssi. C

JavaScript

Event-based, stateful, lacking documentation. Go

The IRC adapter for hubot. JavaScript

Event-based. Go

Plugin system, simple api, arbitrary input stream, debug support. JavaScript

Event based IRC Library with a straightforward API (updated fork of PircBot). Java

Perl6 based extendable IRC client framework.

A group of IRC software authors working to enhance, improve, maintain and standardize the IRC protocol. (source)

An attempt to write an update to the original IRC protocol. documentation (source)

Lists of numerics, modes, ISUPPORT tokens and other protocol details. (source)

Collection of misc IRC protocol documentation.

Statistics around the server software in use on networks today. (source)

A CC0 set of test suites, to ensure IRC message parsers are consistent.

Fast and efficient program to create statistics out of various types of chat logs. PHP Web

Convert images to halfblock ANSI or IRC, with a bunch of post-processing filters. Rust