Our favourite remains WLW, however, since there is little chance of it being released open source, chances are this will continue to be a Windows-desktop-only solution.
A more “universal” approach is Google Docs (now called Google Drive). You can use it with your Gmail account and it looks and feels very much like a text editor, such as Microsoft Word. When done, you can easily share your document (there’s a button in the top right corner) and see changes and comments as they appear, or you can download it as a “zipped Web Page” which can easily be emailed.
Finally, most new browsers allow a “notepad-like” window by entering the following code in the address bar.
data:text/html, <html contenteditable>
That document can be formatted with a number of key combinations (bold is Ctrl+B, italics is Ctrl+I, underline is Ctrl+U, etc) and the document can then be saved from the browser file menu or pressing Ctrl+S.
Sources / More info: Google Drive