pyOCD
A Python debugging and flashing tool for Cortex-M, with chip support pulled straight from vendors' CMSIS packs.
pyOCD came out of Arm's mbed team, which needed a cross-platform way to flash and debug development boards fitted with CMSIS-DAP probes. It is Apache 2.0 licensed and usable both from the command line and as a Python library, which makes it a favourite in test rigs where one script has to flash, run and read back results in the same loop.
The main difference from OpenOCD is where chip knowledge lives. Instead of hand-maintained config files, pyOCD downloads CMSIS-Pack files in which the vendor itself describes the memory map and flash algorithm. That buys support for thousands of Cortex-M variants and none for any other architecture. Probes can be CMSIS-DAP, ST-Link or J-Link, and a built-in GDB server keeps the rest of the toolchain happy.