GtkGuitune is a Linux program for tuning guitars and other instruments by using the method of Schmitt-triggering, i.e. counting the number of triggerings between two trigger levels in a certain amount of time.
Old SW not out-of-the-box compatible with modern distros based on Pulseaudio. Can be however started by running "padsp gtkguitune" via terminal. padsp creates the required /dev/dsp interface.