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.
Greenshot - Greenshot is a free and open source screenshot tool that allows annotation and highlighting using the built-in image editor.
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.
Shutter - Shutter is a feature-rich screenshot program.
Google Keep - Capture notes, share them with others, and access them from your computer, phone or tablet. Free with a Google account.
Snipping Tool - Use Snipping Tool to capture a screen shot, or snip, of any object on your screen, and then annotate, save, or share the image