Bluegrams Screen Ruler - A lightweight and configurable ruler tool for Windows Desktop.
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.
Meazure - What are the dimensions of that image? How big is this window?
OneNote - Get the OneNote app for free on your tablet, phone, and computer, so you can capture your ideas and to-do lists in one place wherever you are. Or try OneNote with Office for free.
On-Screen Ruler - lightweight, free, semi-transparent ruler
Standard Notes - A safe place for your notes, thoughts, and life's work