Padlet - Visual boards for organizing anything.

Raneto - Open source, flat-file Knowledgebase platform that uses static Markdown files

Pinterest - Pinterest is a visual discovery tool that you can use to find ideas for all your projects and interests.

HowHive - Knowledgebases for teams and companies of all sizes. Knowledge sharing and finding made simple.

Pearltrees - Pearltrees is a place to organize all your interests. It lets you organize, explore and share everything you like.

Obsidian.md - A second brain, for you, forever. Obsidian is a powerful knowledge base that works on top of a local folder of plain text Markdown files.