The pentatonic scale is the guitar players best friend. I can barely think of a solo that doesn't use one extensively in the blues rock genre. It is the sound of the blues, but did you know that it is also used extensively in music from many other cultures such as Japan, India, Greece, West Africa, Indonesian gamelan music, as well as a favorite of mine, the Scottish Highland bagpipe. The reason why the pentatonic scale (there are a few) is so popular is that there are no dissonant intervals present. All the notes can be played in unison and sound tonal. Pretty cool huh?