While not obviously related to 3D, design is very much related to 3D. As 3D artists we use the principles of design every day. Everything from form, shape, type, texture and so on. All the way to composition, color theory and line.
This page though is not about that. Instead, I want to list the tools that I have used to complement my 3D work when I needed 2D design elements. I am by no means a designer in the general sense. But these are the tools that help me make the designer job when I need it done.
This is a superb web based tool that helps me generate most social media images that I use. You choose a template and adjust a few parameters and the design is done. Then you just pick the social media channel or use case for your design. Make slight adjustments and download each image.
I have never used a quicker tool for getting basic designs done. This is the "get it done quick" tool. Not the "have full control" tool.
This is my go-to tool for Design. I think it is clear, easy to use and well-balanced. With some basic knowledge you can get going very fast. Affinity designer has tools for both vector art and pixel art.
I view this application as an app that does the basics exceptionally well. Then it leaves out most unnecessary fluff and focus on basics tools.
After all, the basic tools gets us 80% of the way. The rest gets done by combining these basic tools in creative ways to get the result we want.
Before affinity designer, I used Inkscape. A free and open source tool that is excellent to get started with if you don't want to make any money investment. A bit clunkier than Affinity Designer in my opinion. But there has also been some great designs made in this software.
This is not a tool, but a marketplace. It is a good place to browse for inspiration or to get an occasional design template or font. I like it because it is so vast. You can even find 3D models even if it isn't the marketplace I would go to for that kind of content.
This site has a large collection of icons that you can use. They come as .svg files. You can use them in designs, websites or bring them into Blender and extrude them into 3D shapes. What you want really.