pd can do this. :-) the fiddle~ object does pitch recognition (though only for the fundamental frequency), and then plug the midi output to a virtual alsa midi port and then into whatever program you want to use. but i have no idea how accurate/fast/reliable fiddle~ is. i don't know why they don't make guitars (i assume this is for guitar) with a small current on each string and calculate the note from which frets and strings are making electrical contact. you couldn't do vibrato, but that would be a great way to input exact notes. but then you could combine that with the pitch and envelope recognition to get more of the timbre of the instrument. it would just give a definite value for the pitch.