Originally called into existence by Michael*, the Forks are yelled for whenever a mind-scarring image, mental or literal, appears on the Board. The Forks were a very popular running theme throughout 2004-6 but have declined in mention since 2007, possibly due to the comparable decline of the AH.com Series, which often mentioned the Forks.
It should be noted that, while many people think that you're supposed to jab the forks into your eyes, this would still leave the original memory untouched, and Michael has repeatedly pointed out that they're supposed to be inserted directly into the brain.
More recently, the forks has gone out of fashion, to be replaced by the “more efficient” brain-bleach.
* (This meme most probably has an older, non-AH.com origin.)