Notepad in a browser
Type the following into the address bar of your browser to get a scratchpad.
data:text/html, <html contenteditable>
It won’t save your notes, but when you need to jot something down and you have a browser open, this is pretty useful. Note: adding that tag into your page source makes things unclickable, but editable. Take care.
Update: Better versions exist elsewhere. For example, I now have this one bookmarked instead, which uses a better font size, pads the scratchpad over to the right, and adds a title bar. Neat!
data:text/html, <title>Text Editor</title><body contenteditable style="font-size:2rem;line-height:1.4;max-width:100rem;margin:0 auto;padding:2rem;">