Dropbox, Slack, and public key database all in one open source, encrypted program. Encryption for everyone is the driving motivation of the Keybase team, making them a truly admirable start. I still use other services as back up, however -- I'm just that paranoid.
Being able to connect my notes and form a personal, local Wikipedia has been my dream ever since reading Vannevar Bush's "As We May Think." Obsidian makes that possible. It's open source with a lovely community to boot!