"In a grocery store 4,820 canns of Campbell's Soup were used to build an installation piece that spelled HUNGER. Signoge beside the piece encouraged shoppers to buy one and donate it to their local food bank. As shoppers bought cans from the display the word HUNGER slowly disappeared. This allowed people to see how their individual effort could help bring an end to the problem of hunger."
EuroRSCG Amsterdam ad agency put together a very interesting poster for Doctors of the World, which depicts the patient to doctor ratios in different countries around the world.
I spent the evening looking at the SL presence for as many major brands as I could find. I'm sure I've missed a few, but this is a fairly good representation of what real world advertisers are doing in the virtual world space. From a creative standpoint, it's not hard to see why advertisers are so interested in exploring Second Life's possibilities. There aren't a lot of creative or physical limitations inherent in the environment, so it's a unique opportunity to present a brand in an entirely new context.
So, without further ado, here is the massive list of real world brands that are currently participating in the virtual world of Second Life. Enjoy.