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Chrome Device Mode

Firebug

Screenfly by Quirktools - http://quirktools.com/screenfly/

Dimensions Toolkit - https://chrome.google.com/webstore/detail/dimensions/baocaagndhipibgklemoalmkljaimfdj?hl=en
Dimensions Toolkit no longer exists as a simulator. Too bad, because it had a sleek user-interface and many view options. I think it evolved into this Google Chrome tool for measuring screen dimensions. If you find out more, please let me know!

StudioPress Mobile Responsive Design Testing - discontinued

Matt Kersley Responsive Design Testing - http://mattkersley.com/responsive/

isResponsive - http://www.isresponsive.com/

Responsinator - http://www.responsinator.com/

BrowserStack - https://www.browserstack.com/

https://github.com/xcartmods/Responsive-Design-Previewer
Upload preview.php and run in your browser (i.e., place preview.php in file directory)
Go directly to any URL by adding ?url=somesite.com/whatever.html
Demo - https://www.xcartmods.co.uk/preview.php?url=https://getbootstrap.com
Example - https://askdesign2018-dev/preview.php?url=https://askdesign2018-dev/

Visual Inspector - Collaboration tool for website feedback and fixing design bugs. Recommended by Wayne Stratton.
https://chrome.google.com/webstore/detail/visual-inspector-by-canva/efaejpgmekdkcngpbghnpcmbpbngoclc?hl=en-US