ShareX - ShareX is a free and open source program that lets you capture or record any area of your screen...
draw.io - Online diagramming application
Greenshot - Greenshot is a free and open source screenshot tool that allows annotation and highlighting using the built-in image editor.
LucidChart - LucidChart is the missing link in online productivity suites. LucidChart allows users to create, collaborate on, and publish attractive flowcharts and other diagrams from a web browser.
LightShot - The fastest way to take a customizable screenshot.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.