Google announced, Google Checkout Referrals in additon to the exisiting AdSense, AdWords, Firefox with Google Toolbar, and Google Pack referrals. The  Checkout referrals are currently only available for U.S. publishers, every time you refer a user to Google Checkout, you make a little money. It's a simple process three steps:

1. You place the Checkout referral button on your site.
2. Someone clicks on the button, signs up as a buyer with Google Checkout using a valid U.S. credit card, and completes a purchase of at least $10 before shipping and tax through Checkout within 90 days. (The current $10 minimum purchase corresponds to our existing $10 promotion for new buyers, so this amount may change in the future.)
3. You earn $1.

Google Checkout is currently only available for buyers with a U.S. credit card, that gives users a faster, safer, and more convenient way to shop online. Try it out yourself? And, you get a $10 bonus at sign up.

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Google, AdSense, Referrals, Google Checkout Referrals