Guitar and piano are usually both equal tempered, though pianos typically have a slightly stretched octave to compensate for the strings having harmonics that aren't exact integer multiples of the fundamental. (In other words, it's done to compensate for the physical limitations of the instrument, rather than to compensate for the mathematical limitations of the tuning system.)
One can, of course, tune a piano to just intonation. (My piano is currently tuned that way, partly because I wanted to hear what it would sound like, and partly because, with an oscilloscope, it's a lot easier to tune to just intonation than 12TET and I'm too stingy to hire a tuner to do it the "right" way.)
It is possible to make a just intonation guitar as well, though it's not usually done. Here is one of mine that I made a couple years ago. I just finished putting together a just intonation walnut through-neck electric bass with a 2-octave fingerboard a couple days ago.