I have a massive tool. In fact I have lots of them. That just sounds wrong. Anyway, I'm going to list ones I've found handy.

Local CSS Dev

Image placeholders

So we want an image 250px by 250px for testing. There are some cool resources out there. Check out 8 of the top 10 placeholders if you don't like the ones below.

Linting

Jshint was good for a while, but now we use eslint.

System design

Draw.io is an awesome system design tool.

Emmet

If you are doing any front end stuff you really need to know how to use emmet. Checkout this cool Emmet cheat sheet.

Server one-liners

These one liners are essential, runing a python server on one line plus many more.

Cross Browser Testing

browserstack.com

Security

keepassx.org

Mime types

A complete list of MIME types

UX design tools

UI UX resources