Writeboard, which is free, allows a user to create a new document very quickly, password protect it, and add users who can edit the document.
Unlike competitive solutions, you must use a special markup language to format text (no wysiwyg), you cannot upload images, and there is no ajax or other functionality to move content around on the page. It is also a little buggy – comments are not showing up at all on our test page. Frankly, if it wasn’t 37signals, who generally create awesome applications, we would not be profiling it yet.
However, our guess is that they will be adding functionality quickly. We are also looking forward to the launch of Chalk.
SolutionWatch has posted a thorough review of Writeboard.