Chalk - Chalk is a calculator that tries to do right what others do wrong.
Greenshot - Greenshot is a free and open source screenshot tool that allows annotation and highlighting using the built-in image editor.
Public - Group chats.
Shutter - Shutter is a feature-rich screenshot program.
Slaask - Slaask enables you to set up a dedicated live chat channel on your Slack.
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