GtkGuitune is one of the best programs for tuning guitars (and other instruments) in GNU/Linux system.
It is actually an oscilloscope which counts the passes between two trigger levels per time, calculates the frequency and shows the tone on a tone-scale.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
# sudo apt-get install gtkguitune