I've always been a fan of the underground, both art and public transportation. I think the first time every American hears, "Mind the gap" in a station, a little bit of them falls in love with London.
Over the years, the years the Tube has had some pretty great art made in its name, like the Tate Gallery Underground poster above. And now there's a site that lets you check out the art of the London Underground. You can look through current station art, see past collections or even get a sneak peek at future installations. It's worth the trip.