All of them. To be specific, any stand-alone printer that has Google Cloud Print technology built-in is supported without needing an attached computer; and also any printer that you can connect to your computer and print from. (Because, if you can print from your computer, then you can install Google Cloud Print’s driver onto your computer, and then our plugin can print to your printer. Of course, your computer needs to be connected to the Internet and to the printer, in order to be able to receive the print jobs from Google and send them to the printer).
This includes thermal and receipt printers (and a template suitable for narrow printers is included, as well as the ability to customise your templates).
Some printers which don’t have Google Cloud Print built-in (such that they don’t need any other device to drive them) might have a similar system together with the possibility of connecting that system to Google Cloud Print. For example, an HP I once bought came with HP ePrint, and my HP ePrint account could be directly be connected to Google Cloud Print. Though, on the latest HP I bought, it also has Google Cloud Print built in directly as well as HP ePrint.
Of course, whilst Google Cloud Print is a system designed to print to any printer (and has a several-year history of doing so), we cannot warrant this (or anything related – e.g. what’s in the HP you hope to buy) in the case of any particular printer, since it is out of our control; and it is the customer’s responsibility to plan and test any printer purchase. Neither can we comment upon the suitability of any particular printer, unless you are willing to make an arrangement with us to ship it to us for our testing, and cover all the associated costs of doing so (including our time). It’s not realistic for us to take on responsibility relating to printer purchases within the price that the plugin costs!