BAAAAAH! BAH!! BAAAH!!!
The "BAH!"/"Crazy Old Woman" fad is based off an unreleased Xbox 360 ad. In the commercial, an old woman is pretending she's in an FPS game, shouting "BAH" each time she "shoots" as she moves around. The fad was very slow to start up, and eventually was made popular by syncan and MasterSitsu, each with a pair of sites.
Currently, the most popular variation involves adding the woman's "BAH!" to random songs.
Other sites feature the "BAH!" lady in combat, as if her imaginary gun-shots (through her hand) were actually real. Not many people know the fad all to well but at the rate it's picking up, we might just see it in the hall of fame one day!