There's a lot of classic screamers already listed here but few ones haven't been created an existing article as of yet. And also some still do exist until this day like the Madden 07 Subliminal Message YouTube video. The one screamer that interests me though is Yen Imports.
WARNING: There's also pictures of the screamer faces themselves in the articles like in Where's Waldo, The Maze, and in Spanish Castle Illusion. Please browse at them with caution.