Gmail's contextual ads
The display of ads next to the viewed emails (where the ad is based on the content of those emails) is bane and boon. Commercial emailers worry about their emails triggering ads for competitors. But you can also use AdWords to get your ad displayed next to a competitor's email.
Anyway, for a few anecdotal stories about how Gmail's ads work (or don't), check this Wall Street Journal article, which gets some input from those who use the service.
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