Google is in the process of rolling out a new ad system for Gmail, that could prove to be quite powerful: ads that learn what you're interested in based on your email habits.
Here's what Google says:
"If you've been using Gmail for a while, you may have noticed a time or two where we've missed. Bad ads tend to annoy people. We're trying to cut down on these ads, and make the ones you do see much more useful.
[…]Soon we're going to try a similar approach like "Priority Inbox" to ads: using some of the same signals that help predict which messages are likely to be important to you, Gmail will better predict which ads may be useful to you. For e.g., if you've recently received a lot of messages about photography or cameras, a deal from a local camera store might be interesting. On the other hand if you've reported these messages as spam, you probably don't want to see that deal.
Only a few users will notice the change to begin with, but as we improve it we'll roll it out more widely. As always, ads in Gmail are fully automated-no humans read your messages- and no messages or personally identifiable information about you is shared with advertisers.
We've already cut down the number of ads shown per Gmail user by more than a third, and we hope these signals will enable us to continue to show fewer, better ads in Gmail."