What is a must have dev tool that frontend or backend developers should use?

Dash is great. It's often faster to look up docs in Dash than it is to google it. I use it everyday.

Continious delivery system saves your life in terms of spending time on tetsing, dependencies etc

Not only a "dev" tool, but I find this super useful for developers. Paste is a clipboard manager that you can use to save snippets of code and then search/paste them later. I find it really helpful when I'm thinking "what was that code snippet I used 2 weeks ago, but didn't save anywhere"

TemplateToaster is a powerful theme generator, used for designing themes and templates for five major content management systems. WordPress, Joomla, Drupal, Magento, and Prestashop. Add to this, it is also used for creating Blogger templates.

This is must-have for front-end developers. It gives you debug-friendly user sessions.