What is a flanger?
A flanger effect is produced when the sound from your guitar is mixed with a copy of your signal that is delayed by a small and gradually changing period. When this signal is mixed with the original signal the frequency spectrum is changed in peaks and notches which produces a sweeping sound. The two signals kind of moves around and relates to each other in a linear harmonic serie. The flanger sound can often be described as a "jet plane" or a "swoosh".
Any good examples?
An early example of flanging can be heard on the rocket launch sound effect on The Ventures version of the song "Telstar". John Lennon became a fan of flanging when he got tired of double-tracking his vocals and engineer Ken Townsend solved the matter by inventing something he called Artificial Double Tracking, which was a kind of flanger. Lennon loved the sound and tried it on many tracks, the first one in the Beatles catalog is "Tomorrow Never Knows". Eddie Kramer did the fisrt stereo flanging on the coda, the fantastic outro solo, on the Jimi Hendrix song "Bold As Love". Jimi stated that this was the sound he hd heard in his dreams. Another famous user of flanging is Eddie Van Halen which can be heard on several Van Halen tracks, a good example is "...And THe Cradle Will Rock". Other examples of a flanger can be heard on songs like "Animal Nitrate" by Suede and the breakdown section before the solo in the Lenny Kravitz song "Are You Gonna Go My Way".
Which pedal should I get?
The two Boss pedals BF-2 and BF-3 are classics, but although their names are similar there´s many differences betwen these two pedals. To use an easy explanation the BF-2 sounds analog while the BF-3 is a digital unit. For the Van Halen-fan there´s an easy choice - the MXR EVH117. Another guitar hero with his own signature flaner is Paul Gilbert with his Ibanez AF-2 Airplane Flanger. Electro Harmonix has their own flagship in the Electri Mistress Flanger, famously used by David Gilmour on Pink Floyd´s classic album "The Wall". And ther´s of course more options out there, so go on and try a few before you make your decision. It´s your ears that decides which flanger that suits you best.