Google on the AdSense blog has announced referrals for Google Apps. Google Apps, has taken, Google's most powerful communication and collaboration tools, and packaged together so businesses, schools and other organizations can work more effectively while spending less time and money on IT systems. Google Apps provides customized email addresses (powered by Gmail), Google Calendar, Google Talk, Docs & Spreadsheets and Page Creator, all for free. Best of all, everything is hosted by Google, so there's no hardware or software to download, install or maintain.
To setup Google Apps referrals, 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 currently available in English only, other languages will get add soon. Finally, copy the code provided into the design of your web pages!
Note: It seems, Google Apps referrals to be US specfic only, as I couldn’t see any button for Google Apps. All that I could see referrals for AdSense, AdWords, Firefox + Google Toolbar, and Google Pack.
When someone clicks the referral ad from your site and signs up for Google Apps with a domain that has not already been signed up for the service, you will be credited $5. That person's domain must then have one or more Google Apps email accounts in use for four consecutive weeks. It's that simple.
If you're new to AdSense referrals, you might want to check referrals optimization tips.
Source:? AdSense BlogGoogle, AdSense, Referrals, Google Apps, Announcement