Pidgin - Pidgin is an easy to use and free chat client used by millions. Connect to AIM, MSN, Yahoo, and more chat networks all at once.
Adium - Adium is a free instant messaging application for Mac OS X that can connect to AIM, MSN, Jabber, Yahoo, and more.
Trillian - Trillian is a decentralized and federated instant messaging platform that lets your whole company send private and group messages, keep tabs on what co-workers are doing, share files, and much more.
Franz - Franz is your messaging app for WhatsApp, Facebook Messenger, Slack, Gmail, Telegram and many many more.
Skype - Stay in touch with your family and friends for free on Skype. Download Skype today to chat and call on desktop and mobile.
Rambox - Digital workspace organizer that allows you to unify as many applications as you want, all in one place. It is perfect for those who care about productivity while working with many business and personal apps.