ST-Link
STMicroelectronics' own debug probe for STM32 and STM8 — the one most people own without ever having bought it.
A standalone ST-Link costs a fraction of a J-Link, but the commonest version is built into ST's Nucleo and Discovery boards: the snap-off top section of the board is a complete probe. That on-board version also exposes a virtual serial port over the same USB cable and a mass-storage drive you can drop a binary onto to flash it.
It speaks SWD and JTAG to STM32 and ST's single-wire SWIM protocol to the 8-bit STM8. ST-Link/V3 arrived in 2018 with higher speeds. Chinese clones in USB-stick form sell for pocket change and mostly work, right up until an ST firmware update notices them. OpenOCD, pyOCD and probe-rs all support it, and SEGGER has long offered firmware that turns the on-board variant into a J-Link.