Yazi, If, and Ranger file managers use it to display images inline. Euporie allows you to run a graphical Jupyter console session.

Sixel is a very old standard and has been used to display graphics in terminals since the 80s. It fell out of favor when high-resolution bitmapped desktop environments became mainstream (and affordable), but it is still useful if you are doing remote console work and don't want to set up network hoops with ad hoc HTTP servers just to view a remote diagram.