OpenOCD

Dominic Rath wrote it as his diploma thesis at the University of Applied Sciences in Augsburg in 2005, targeting ARM7 and ARM9 cores. It has since grown to know hundreds of chips and most probes — FTDI-based adapters, ST-Link, J-Link, CMSIS-DAP — and it is what runs under the hood of a surprising number of IDEs that claim to have their own tooling.

In use it opens a GDB server on port 3333 and a telnet console on 4444, and it is configured with Tcl scripts describing the probe, the chip and the board. It can flash, single-step, read memory and service semihosting calls. Its error messages are notorious, but for years the alternative was a proprietary debugger per vendor.