Not just fretted instruments, pianos as well. Anything which lacks ability to adjust tuning on its own, while being played (things that wind instrumenst and non-fretted strings can do), for example a steel pan drum, would be equally tempered (assuming you want to be able to play in all possible keys).

>That's why symphony orchestra players will often have a different flute, clarinet, or oboe for different tunings. Not sure what you are referring to here? Clarinets don't come in different "tunings" unless you mean different keys - like Eb or Bb clarinet, but those aren't there for intonation, they play in different ranges of pitch than one another.

Edit: in addition, you don't HAVE to equally temper a guitar. You can choose just intonation. The problem is that you can only have just intonation on a single KEY for that instrument. So if you tune justly to C major, a key like B major, will sound horrendous!