When it comes to expressive playing, no player should be without a wah pedal. Originally used as a effect for horn players in the 1920's by moving a mute around the bell of the instrument, the wah sound of today is essentially a sweeping filter effect created by using a potentiometer to sweep the EQ curve. This effect was brought to popular music in a big way with the introduction of the wah-wah crybaby pedal in the sixties and was evangelized by several notable artists, including Jimi Hendrix who used the pedal as an instrument to create amazing and dynamic rhythms. It could be stated that popular music wouldn't have taken the same path had it not been for the wah pedal. It was and still is that important.
In the last few years, the auto wah pedal has also become quite popular as it will automatically generate a wah sound according to the dynamics of your playing.
Today, the wah pedal is a main attraction for soloists of all kinds. Have a listen to any Metallica or Guns n' Roses album and you'll be sure to hear the wah pedal in all its glory, not to mention countless other artists from all different genres of music.