ST-Link

There's a number of different ways to flash STM32 devices. One of these is to use ST's own STLink devices.

There are multiple benefits of using one of these STLink devices rather than a plain serial dongle:

1. The device need not be restarted in flash mode (Boot0)
2. Programs can be debugged on the device

The official STLink as seen above are relatively cheap (< $20) and there are Chinese copies available (haven't tried those).

The most common option are these:

They are Chinese knock offs and they run the official STLink firmware.