The initial version of WYSIWYG editor based on QtWebKit (see my post on Qt Labs for details) apparently works pretty well. There are rough edges here and there, something which need fixing, but overall I am quite happy. Live editing a web page opens a whole new possibilty. For an HTML-based help system, you can offer annotation feature, where the user can add his own note right inside the documentation (you may want enable editing on certain parts, not the whole document). You can make a note-taking system that copies something from the web (in HTML) and then the user can touch the content to adjust it to his need. Translate web pages easily, with all the formatting and whatnot intact, and without the need to scare the translators with raw HTML black magic. Any more useful examples?
Screenshots follow. Click on each image to enlarge.