Web Development Tools
Some tools that will make your life as a web designer or developer easier.
Graphic Tools
(Windows) PNGGauntlet - Even now with most people having high-speed Internet connections, it's nice to remember that some people still have to deal with limited bandwidth. Making PNG files as small as possible is a good place to start.
I use this free program all the time to minimize the size of my images.
(Mac OS) ImageOptim
Site Development Tools
(Windows, Mac, and Linux) Visual Studio Code - Don't let the fact that this tool was created by Microsoft put you off. I avoided it for a while because I thought it was just a trimmed down version of Visual Studio. It isn't. It is an extremely useful tool, and not only for web development. It is well-integrated with Git, making source control much easier than a lot of other tools I've tried.
Debugging Tools
Your web browser makes a good debugging tool for most web development work. All of today's major browsers have some sort of debugging tools.