Simpleaf is a very simple and surprisingly efficient handwriting memo app.
For example, when you want to make a note while web browsing or making phone calls, you simply swipe down the status bar and launch Simpleaf.

You can also make a note, at the same time you are looking at the screen by making Simpleaf translucent.

Notes can be transmitted to mail-apps or Evernote and so on.
(Traffic can be saved by specifying an image size and format.)

This free version is supported by ads.
and
The Simpleaf logo appears in the lower-right corner of the shared image.
(Subtle advertising on Twitter and Facebook)