Here are links to sites that have been useful to me. They concern Cortex-M microcontrollers, development tools, and discussions on subjects that are sometimes very specific.
Getting started guides
Step by step blinky application on STM32: https://gitlab.com/stm32mcu/wiki/-/wikis/home#blinky-the-embedded-hello-world
Some STM32 subtleties to know
A good connoisseur of STM32 (among others) has compiled a set of remarks and information following his intense activity on the forums. Take advantage of it: STM32 gotchas
If you are an old hand you will also be interested in this who’s who!
STM32 Nucleo Boards
STMicroelectonics presents more than 65 Nucleo boards. These low-cost boards use most STM32 MCUs, from the STM32L0 series based on Cortex M0+ to the powerful STM32H7 series based on Cortex M7.
AdAstra-RTK BSPs are available for several of these modules.
The STM32 Nucleo boards integrate an ST-Link debugger/programmer, so there is no need for a separate probe.
Free IDE Based on ECLIPSE™/CDT, with support of ECLIPSE™ add-ons, GNU C/C++ for Arm® toolchain and GDB debugger : TrueStudio IDE
This site offers good tutorials on STM32CubeIDE and STM32CubeMx development tools.
There is also a STM32 quick start guide and tutorials, and other useful libraries.
A good place to start with Eclipse and STMicroelectronics STM32 MCUs. There are several interesting tips. The free manual explains step by step how to configure Eclipse, to develop, flash and debug an application.
Libraries and tutorials for STM32Fxxx series by Tilen Majerle. Mainly for the STM32F4 series, but many examples can be ported to other series. Anyway clear and simple examples for the use of peripherals are always good to have.
GNU MCU Eclipse
A family of Eclipse CDT extensions and tools for GNU ARM & RISC-V development
MCU on Eclipse
A blog with a huge amount of information about ARM Cortex-M (mainly NXP MCU).