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, Evernote and so on.
(Traffic can be saved by specifying an image size and format.)