Pepsiacc
Pepsi is deploying these posters in Toronto and Vancouver subways, when your plug your headphones in they play music – Link. You could make your own version by grabbing one of these $9 MP3 players, hacking it up and extending the battery….Could be fun for a DIY art frame that plays music or talks when you plug in to it, or a lot of other things….

Update: These Coke posters scream at people too.