Open-source tool for writers
Minder
Mind-mapping application
RetroShare
Retroshare establish encrypted connections between you and your friends to create a network of computers, and provides various distributed services on top of it: forums, channels, chat, mail…
Evolución
Evolution is a personal information management application that provides integrated mail, calendaring and address book functionality.
Notes-Up
Notes Up is a notes manager written for elementary OS. With it, you’ll be able to write beautiful notes fast and easy using the markdown format.
Notable
The markdown-based note-taking app that doesn’t suck.
Heimer
Heimer is a desktop application for creating mind maps and other suitable diagrams.
GNote
Gnote is a desktop note-taking application for GNOME.
Thunderbird
Thunderbird is a free email application that’s easy to set up and customize – and it’s loaded with great features!