Any instrument where the pitch is not quantized into steps by design.

Guitars do this with frets, though there are of course fretless guitars but the problem is the quantization of pitch/fretted strength length that frets provides allows complex chords to be played in tune. This is functionally impossible to do on a fretless stringed instrument and is why violin type instruments mostly play single notes or two paired notes.

A classic church organ is the bridge between the world of quantized pitch instruments and instruments where pitch is constructed as part of each note. Each pipe produces a sine wave at a determined pitch.