The HPLIP project provides free software printer drivers for GNU/Linux machines. Here is a list of supported printers. Be careful using this list because a small percentage of these machines require proprietary firmware in addition to the free driver. Any printer with "yes" in the far right column requires a non-free "driver plug-in" and so could not be part of a fully-free system.

You might also find useful information at linuxprinting.org. Note that they list many printers that require nonfree software to work as well (but are compatible with GNU/Linux systems using proprietary drivers), so be careful.

If you have information about printers that work or don't work with free software, please let us know!