Greenshot - Greenshot is a free and open source screenshot tool that allows annotation and highlighting using the built-in image editor.

Slack - A messaging app for teams who see through the Earth!

Shutter - Shutter is a feature-rich screenshot program.

Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.

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

Mattermost - Mattermost is an open source alternative to Slack.