Google AdSense blog informs that a referral to Google Apps can earn you $5. However, it seems to be restricted for US users alone.

Instructions given on AdSense blog:
Just visit the AdSense Setup tab in your AdSense account and click the link for Referrals. Select the Google Apps option and then choose how you want the referral ad to look on your site. (The Google Apps referral ads are available in English right now, but we'll be adding more languages soon.) Copy the code provided into the design of your web pages, and voila!

If you're new to AdSense referrals check out referrals optimization tips.

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