ST-LINK/V2 'clone' or 'mini' devices are cheap and compact programmer/debugger devices. Their schematic and firmware are based on the ST-LINK v2 by ST Microelectronics, but they have a more compact 'USB stick' form factor.
They run a STM32F103CBT6 microcontroller, or a cheaper variant which is still compatible.
They can be considered the cheapest and most compact STM32 boards available, but have a very low pin count.
It is possible to re-flash this device with another application or another bootloader. To do this, you need to solder wires or pin headers on the PCB.
For example, the debugger firmwares of Black Magic Probe and CMSIS-DAP have been ported to run on the ST-LINK.
Many (cheap to ultracheap) variants exists of the ST-LINK clones. Pinouts may differ between these models. Most (all?) of these contain the bootloader and firmware of the official ST-LINK v2, which is copied without permission. In general, updating their firmware to the latest official release is possible using software provided by ST Microelectronics.
Some clones use an F103C8 microcontroller instead of F103CB, so they have (oficially) less flash memory. In practice, they can still contain the full 128 kB. Other clones use an F101CB, which (oficially) lacks USB support. They still contain and use hardware USB functionality though, it's just not officially tested and guaranteed by ST. Pinout may differ between different clones.
Clone ST-LINK adapters come in colored aluminium casings, plastic casings, plastic wrapping, or as bare PCBs. Other form factors are also possible -- some have a USB micro-B socket instead of the regular USB A plug.
They can be found all over eBay and AliExpress. Adafruit also carries them.