Both official and unofficial tools that can make working with QMK easier.

QMK Configurator

Awesome tool for anybody who does not need advanced funtionality, just an easy way to work on keymaps. -> mainline QMK converter

Behind the scenes, uses an ancient fork of QMK (3+ years old, afaik).
Tries to bring the code generated by up to standards.
It’s a great starting point to add support for boards that has kbfirmware json but not supported by mainline QMK.

VID/PID usage info

Vendor and product IDs become a topic of conversation with VIA, since it uses these two values to indentify a board.
Pulls the usage data from the QMK API and allows filtering.
Useful for those thinking about adding VIA support for their boards.

Helix font editor

Allows you to upload a glcdfont.c file, edit it and save it back to C file. Useful for OLED-lovers.

Logo editor

Another useful tool for users of OLED screens.

Combo generator

Handy tool for generating Combos.