We're excited to announce that now, in addition to AdSense and Firefox, you can refer users to AdWords in countries and languages where AdSense for content is available. You'll earn $20 once the advertiser you've referred spends at least $100 with AdWords. To get started, choose 'Referrals' on your AdSense Setup tab and look for the AdWords sub-tab.

For more details on referrals, visit the AdSense Help Center.

UPDATED for accuracy