While there are plenty of notepad add-ons for different web browsers that lets you jot down short notes without opening a dedicated notepad application, here is a nifty trick that lets you create quick notes from within your web browser using neither an add-on/extension nor a third-party program.
Just type the following code into the browser's URL bar and press the Enter key.
data:text/html, <html contenteditable>
This turns your browser into a blank HTML page with editable content. Just type your note on the blank page and then save the note – by pressing Ctrl+S - as an HTML file. In Firefox, you can choose to save the page as a regular Text file.
Of course, this is a plain text file so formatting is allowed – things such as bold and italics. Other stuff such as inserting images, bullets etc. are also off-limits. You do, however, get spell checking, thanks to the built-in spell checker that modern web browsers have.
The trick reportedly works on Chrome, Firefox as well as on the old Opera running on Presto engine. It might not work on Internet Explorer.
You can have the code above saved as a bookmark, so that you can quickly create notes with a click whenever you need it.