Mozilla Thunderbird is free and open-source email client software which also functions as a full personal information manager with a calendar and contactbook, as well as an RSS feed reader, chat client (IRC/XMPP/Matrix), and news client. Available cross-platform, it is operated by the Mozilla Foundation's subsidiary MZLA Technologies Corporation. Thunderbird is an independent, community-driven project that is managed and overseen by the Thunderbird Council, which is elected by the Thunderbird Community. The project strategy was originally modeled after that of Mozilla's Firefox Web browser and is an interface built on top of that Web browser.

Eudora is an email client that was used on the classic Mac OS, Mac OS X, and Microsoft Windows operating systems. It also supported several palmtop computing platforms, including Newton and the Palm OS. Eudora was succeeded by Eudora OSE. In 2018, after being years out of print, the software was open-sourced by the Computer History Museum.