I'm not a business person, but I take great delight leafing through Dun and Bradstreet's Who Owns Whom. You can learn so much from it.

For example, where I'm from, there are two major pharmacy chains: Shopper's Drug Mart and PharmaPlus. According to Who Owns Whom (North American edition, 1997), the parent company of PharmaPlus, The Oshawa Group, owns a number of companies mostly having to do with food. They own IGA Canada for example. In contrast, the parent company of Shopper's Drug Mart, Imasco Ltd., owns such companies as American Tobacco of Canada, General Cigar Co., Imperial Tobacco, du Maurier, and Player's Ltd. Hmm... how much does Shopper's really care about my health?

2000 update: 
Imasco no longer exists
 

Here's another example that works on both sides of the border:

Did you know that you've been
buying foods from a tobacco company?

I don't smoke, myself. While I'm no anti-tobacco zealot, I definitely have misgivings about tobacco companies and I do what I can to avoid supporting them. But as hypocrisy is the greatest luxury, I can't give up Harper's Magazine just yet.