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
STM32 Nucleo Boards
STMicroelectonics presents more than 55 Nucleo boards. These low-cost boards use most STM32 MCUs, from the STM32L0 series based on Cortex M0 + to the 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
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).