Zim - A Desktop Wiki
On this page you find some screenshots of zim in action. Of course pictures only capture the look, not how it feels to use, see the screencast for see a bit more of the feeling.
Screencast thanks to ProductiveLinux.
The editor window with all widgets visible
Minimalistic editor window with a checkbox list and some links
Showing "open with" item for embedded image
Showing the calendar dialog
Showing the alternative embedded calendar view
Showing the task list dialog
The version control dialog
Showing the latex equation editor