Mumble
A low-latency, high quality voice chat program for gaming
Quassel IRC (Client only)
Distributed IRC client with central core component
Quassel IRC
Distributed IRC client with central core component
Smuxi
Chat with other people on IRC
TorChat
Decentralized instant messenger built on top of the Tor Network

QHaveDate
Optimize your probabilities to have sex on dating websites
PCManX BBS Client
PCManX BBS Client

Ricochet IM
Anonymous instant messaging
TinyFugue 5
TinyFugue MU* Client (v5)
Dino
Integrated MIDI piano roll editor and sequencer engine

Riot
A feature-rich client for Matrix.org
ScudCloud
ScudCloud is an open-source client for Slack

qTox
Qt 5 based Tox instant messenger for secure communication
Orion
A Twitch.tv desktop client
Jitsi
VoIP and Instant Messaging client

Telegram Desktop
Official desktop version of Telegram messaging app

Weechat
Extensible chat client
Srain
Modern IRC client written in GTK.