ZimWiki is an open source desktop WYSIWYG text editor written in Python (originally Gtk2-Perl) that runs on Linux, BSD, Win32, and OSX.
From the manual:
""A "desktop wiki" means that we try to capture the idea of a wiki, not as a webpage but as a collection of files on your local file system that can be edited with a GUI application. The main focus is a kind of personal wiki that serves for all kind of notes: todo-lists, addresses, brainstorm ideas etc.
But we want to go further then just a wiki filled with random content. It should also be possible to use your random notes as the basis for more structured data: articles, presentations etc. Zim will not include tools to layout a presentation or something like that, you should use your office suite of choice for that, but it should be a tool that can deliver all the content for a presentation in a form that only needs a template and some layout before usage. Therefore features normally not found in wikis will be added.""
See http://zim-wiki.org/ for more information.