This is a list of web-based tools I wrote.

4bit

This is a tool that helps you generate 4bit greyscale bitmaps with dithering. You can download the resulting image as a header file, to be included in your code.
https://p3dt.net/4bit/
The source code is here:
https://github.com/pauls-3d-things/4bitbitmapbuilder

u8g2sim

The u8g2 library for arduino is great. Unfortunately it takes ages to flash an arduino, so I wrote a simulation tool that speeds up development a bit.
https://p3dt.net/u8g2sim/
The source code is here:
https://github.com/pauls-3d-things/u8g2-simulator

espconfiggen

If you want to quickly generate the boilerplate code required to load/edit/store a few config values with an esp8266 this tool might be for you.
https://p3dt.net/espconfiggen/
The source code is here:
https://github.com/pauls-3d-things/espconfiggen