I love this kind of stuff! When I was a young (right before & after learning to read) kid I used to spend hours looking at National Geographic magazines provided by my Aunt Mildred and Uncle Bernard. I was fascinated by the ads, and for some reason, the Union-Carbide ads were particularly appealing.

This high quality archive of advertisements bring back some of those memories, plus they provide a unique perspective of America during the 1st half of the 20th century.

I found a similiar site a while back. Adflip extends the idea by providing an archive of ads from the 1950's on.