Meazure - What are the dimensions of that image? How big is this window?
Evernote - Bring your life's work together in one digital workspace. Evernote is the place to collect inspirational ideas, write meaningful words, and move your important projects forward.
Bluegrams Screen Ruler - A lightweight and configurable ruler tool for Windows Desktop.
Standard Notes - A safe place for your notes, thoughts, and life's work
On-Screen Ruler - lightweight, free, semi-transparent ruler
Joplin - Joplin is a free, open source note taking and to-do application, which can handle a large number of notes organised into notebooks. The notes are searchable, tagged and modified either from the applications directly or from your own text editor.