What is Auto-Wah?
Auto-Wah, also know as Q-wah, T-was or Envelope filter, is kind of having a wah-wah pedal going all the time but without having to rock one foot on the pedal. You simply switch on the pedal and the wah is on and responses to the volume and attack from your guitar signal. The name of the effect pretty much says it all - it's an automatic wah pedal.
Any good example?
Funk and disco from the seventies is filled with auto-wah, not rarely on other instruments than the guitar. Electric piano, clavinet and bass has often been hooked up with this effect and the result is really funky, listen to the claivnet on the Stevie Wonder song "Higher Ground" for a great example. Grateful Dead is a band that's not really defined as funk, but that didn't stop Jerry Garcia from using auto-wah on several songs in the late seventies such as "Estimated Prophet". The Edge of U2 used the effect on "Mysterious Ways", Pete Townsend of The Who relied on it while recording "Going Mobile" and Mark Knopfler of Dire Straits used an auto-wah on "Industrial Disease". And of course the funk-rockers Red Hot Chili Peppers has also used this effect - Flea on the bass and John Frusciante on the guitar. Cool examples from this group includes "Sir Psycho Sexy" and "Get On Top". The auto-wah also works well in reggae music which can be heard on a lot of Bob Marley songs like "Jammin'".
Which pedal should I get?
There are many fun options out there, some of them are the Electro Harmonix Riddle Q Balls Envelope Filter, the Boss AW-3, the Danelectro French Fries, the Mad Professor Snow White AutoWah, the DOD Envelope Filter 440 and the MXR Auto Q Envelope Filter. Some of them are cheap, other more expensive, and of course with different qualitites and possibilities. Bear i mind that an auto-wah can never replace a regular wah-wah pedal, it's more like a complement that can be used in different ways than a regular wah-wah.