Zim is a WYSIWYG text editor. It aims at bringing the concept of a wiki to your desktop. For example every page is saved as a text file with wiki markup.
Pages can contain links to other pages, and are saved automatically. Creating a new page is as easy as linking to a non-existing page. Pages are ordered in a hierarchical structure that gives it the look and feel of an outliner.
This tool can be used to keep track of TODO lists or ideas, to take notes during a meeting or to draft any other kind of text (blog entries, important mails, etc.).