Indigrid - Plain text outliner with side-by-side views.
DokuWiki - DokuWiki is a simple to use and highly versatile Open Source wiki software that doesn't require a database.
metapad - Metapad is a small, fast (and completely free) text editor for Windows 9x/NT/XP/Vista with similar...
TiddlyWiki - a non-linear personal web notebook
Notion - All-in-one workspace. One tool for your whole team. Write, plan, and get organized.
MediaWiki - MediaWiki is a free software wiki package written in PHP, originally for use on Wikipedia.