USB to UART/TTL adapters
These are common to use for serial access and debugging on embedded devices and single board computers (SBC). They usually come in two variants, as a preassembled cable with dupont/jumper wires at the end, or as a small board with solder points or pre-installed headers.
Common brands of controllers are Cypress, FTDI, Prolific, Silicon Labs and WinChipHead. Except for evaluation boards / cables they aren't usually the manufacturer of the finished product.
Driver support is available for most FTDI, Prolific, Silicon Labs and WinChipHead controllers. However, depending on your application, some variants might be of more use than others.
Older controllers might not be able to handle speeds above 1Mbaud, which some devices or SoCs (such as Rockchip RK3399) utilize, so make sure to verify if that's of importance. In general the difference in price is small so if you're looking to aquire a new adapter you might as well go for one that supports higher speeds.
Controllers that support speeds above 1Mbaud:
FTDI FT230(X*), FT231(X*) and FT232-series (excluding BQ or BL variants) work well and they all support up to 3Mbaud
Prolific PL2303** series
Silicon Labs CP2102N (only N variant) and CP2104
WinChipHead CH340* or CH341*
Controllers that only support speeds below 1Mbaud:
- FTDI FT232BQ and FT232BL (these are very old chips unlikely to be found in modern cables)
- Silicon Labs CP2102 (non N)
Buying a serial adapter
Pricing is usually around 4-20 USD depending on controller and if you want a value model Prolific and Silicon Labs are usually cheaper than FTDI based variants. TTL interfaces uses either 1.8V, 3.3V or 5V and you will damage your device and/or controller by using the wrong one, some are hardset and some have a toggle to switch between voltages (usually 3.3V and 5V).
A good value controller is Silicon Labs CP2104 (soon EoL) which can be had for about 4 USD. (Products below are not verified in any way)
Another controller that works well and is relatively inexpensive is Prolific PL2303TA, however these are EOL (end of 2020) and there are some reports of counterfeit parts.
Be aware that both FTDI FT232R/RL and Prolific PL2303HX/HXA (EOL, look for something else) occasionally have been counterfeit so you might want to consider looking at another controller (model) unless you have a reliable supplier and price seems reasonable (a FTDI chip alone is around 3-4 USD).
Some bugs found in Silicon Labs 2102N https://www.eevblog.com/forum/projects/psa-do-not-use-the-cp2102n-usb-to-serial-converter-in-your-designs-(for-linux)/
You might also want to avoid WinChipHead CH340/CH341 given the comments in the driver https://cgit.freebsd.org/src/tree/sys/dev/usb/serial/uchcom.c#n63
Usually they're true "plug 'n play" devices, in most cases you only connect TX(D), RX(D) and GND, VCC is left unconnected.