Open Source Comms

HexChat is an IRC client based on XChat for Linux , but unlike XChat it’s completely free for both Windows and Unix-like systems.
HexChat was originally called XChat-WDK which in turn was a successor of freakschat.

Features

Easy to use and customizable interface
Cross-platform on Windows, OS X, and Unix-like OSes
Highly scriptable with Python and Perl
Translated in multiple languages
Fully open source and actively developed
Multi-network with auto-connect, join, and identify
Spellcheck, Proxies, SASL, DCC support and more

Mastodon is a decentralized microblogging engine.

FEATURES

• Free and open-source.
• Decentralized, no worries about a single company controlling your communications.
• Supports the standardized ActivityPub protocol
• Fully interoperable with GNU social and many other ActivityPub or OStatus platforms
• Real-time timeline updates
• Media attachments
• OAuth2 and a straightforward REST API
• Background processing for long-running tasks
• Deployable via Docker

Matrix defines a set of open APIs for decentralised communication, suitable for securely publishing, persisting and subscribing to data over a global open federation of servers with no single point of control. Uses include Instant Messaging (IM), Voice over IP (VoIP) signalling, Internet of Things (IoT) communication, and bridging together existing communication silos - providing the basis of a new open real-time communication ecosystem.

Thunderbird is a free, open source, cross-platform e-mail, news and instant messaging client. The project strategy is modeled after Mozilla Firefox.

It supports POP (storing mail locally on your hard drive so that it can be accessed without an internet connection), and IMAP. It features excellent mail filter capabilities and management.

Thunderbird has strong support for using multiple accounts and identities, including automated signature features.

Chat functionality is built in, so you can enjoy real-time conversation with your contacts, right from your favorite messaging application, with multiple supported networks. Thunderbird makes it easy to search through both past conversations and received emails.

Thunderbird can also function as an RSS/Atom feed reader, and has a large selection of add-ons that extend it with calendar support, PGP support, integration with online services and much more.

Available also as portable app: http://portableapps.com/apps/internet/thunderbird_portable