Gajim
Gajim is a free and open source, multi-platform, easy-to-use, and full-featured XMPP client
Mailspring
Mailspring – free and open source, multi-platform fast desktop mail client
Nuvola Player
Nuvola Player – free and open source linux desktop music player for streaming services such as Spotify, YouTube Music, Apple Music and others
KMail
KMail is a free and open source email component of Kontact, an integrated personal information manager from KDE
Minder
Minder is a free and open source software for creating mind maps
ColorMate
ColorMate is a free and open source small app for converting color between three notations #rgb, rgb() and hsl() for Linux
Sylpheed
Sylpheed is a free and open source, multi-platform, simple and lightweight email client
Evolution
Evolution is a free and open source application for managing personal information: mail, contacts, calendar, tasks, and notes
gmusicbrowser
gmusicbrowser is a free and open source software for listening to audio, a music player for large collections
Lollypop
Lollypop is a free and open source music player and music collection organizer