VeraCrypt - Disk Encryption

VeraCrypt

VeraCrypt is a source-available freeware utility used for on-the-fly encryption. It can create a virtual encrypted disk within a file or encrypt a partition or the entire storage device with pre-boot authentication. VeraCrypt is a fork of the discontinued TrueCrypt project. It was initially released on June 22, 2013. According to its developers, security improvements have been implemented and issues raised by the initial TrueCrypt code audit have been addressed.

OS: Windows, Mac, Linux.

GNU Privacy Guard - Email Encryption

GnuPG

GnuPG is a GPL Licensed alternative to the PGP suite of cryptographic software. GnuPG is compliant with RFC 4880, which is the current IETF standards track specification of OpenPGP. Current versions of PGP (and Veridis’ Filecrypt) are interoperable with GnuPG and other OpenPGP-compliant systems. GnuPG is a part of the Free Software Foundation’s GNU software project, and has received major funding from the German government. GPGTools for Mac OS X.

OS: Windows, Mac, Linux, Android, BSD.

PeaZip - File Archive Encryption

PeaZip

PeaZip is a free and open-source file manager and file archiver made by Giorgio Tani. It supports its native PEA archive format (featuring compression, multi volume split and flexible authenticated encryption and integrity check schemes) and other mainstream formats, with special focus on handling open formats. It supports 181 file extensions (as of version 5.5.1).
Mac alternative: Keka is a free file archiver.

OS: Windows, Linux, BSD.