WikiPress features a frontend Wiki Page editor.
Any Wiki Page can be edited by selecting the edit link at the top right of the screen. This will bring up the WordPress post editor, where you can edit the page and preview how it will look to your visitors in near real-time.
Note you must be logged in to edit, though the WikiPress demo features a demo mode that allows you to experience this functionality without logging in (Saving and media uploads are restricted in this demo).