OpenSSL - OpenSSL is a free and open source software cryptography library that implements both the Secure Sockets Layer (SSL) and the Transport Layer Security (TLS) protocols, which are primarily used to provide secure communications between web browsers and …
ClearOS - A very good alternative for Windows Server, for free
Let's Encrypt - Let’s Encrypt is a free, automated, and open certificate authority brought to you by the Internet Security Research Group (ISRG).
NethServer - NethServer is an operating system for Linux enthusiasts, designed for small offices and medium...
Authy - Best rated Two-Factor Authentication smartphone app for consumers, simplest 2fa Rest API for developers and a strong authentication platform for the enterprise.
Univention Corporate Server - Univention Corporate Server UCS with Active Directory functions and App Center is the cost-efficient and popular successor to Microsoft's server solutions.