Exim

Fast, scalable, enterprise mail/groupware server providing ESMTP, IMAP, POP3, webmail, mailing list, basic web-based calendaring and scheduling services.

Fast, light and extensible chat client.

Easy to use but advanced IP PBX system, VoIP gateway and conference server.

IMAP and POP3 server written primarily with security in mind.

Software framework for decentralized, peer-to-peer networking.

General-purpose, lightweight, minimalist WebRTC Server.