I think Passion of the Christ might take the cake for biggest forgotten cultural phenomenon of the 2000's. Literally EVERYONE was talking about it (even some of my classmates in 2nd grade

) and it made insane amounts of controversy paralleled by none, but I never hear anything about it anymore.
Juno is up there too, but the movie is so dated and lame, so I can see why people don't go back to watch.