In Optimization Tip 3: Increase earnings with other Adsense features, you've seen how referring users to Google products and services can increase your Adsense income. Just place a referral link in your site and every time a visitor downloads Google-endorsed programs or joins one of Google's services, you earn credit depending on the user's location.
Here's some more info about these Google products and services being offered in the Adsense Referral Program.
Firefox – Mozilla Firefox is a web browser with pop-up blocking, tabbed browsing, and privacy and security features. It also integrates the Google Toolbar which offers users Google search, SpellCheck, and Autofill. When a user you've referred to Firefox plus Google Toolbar runs Firefox for the first time, you will receive up to US$1.00 in your account, depending on the user's location. The referral must be a Windows user who has not previously installed Firefox in order for you to receive credit.
Picasa – Picasa is a Windows photo management software with advanced editing and picture sharing capability. When a user you've referred to Picasa downloads and runs the product for the first time, your account will be credited with up to US$1.00. The user you refer must be using Windows and not have previously installed Picasa in order for you to receive credit.
Google Pack – Google Pack provides a single installer for many essential PC applications from Google and other trusted partners. These applications improve browsing with increased speed and security, provide better searching capability for the web and your PC, and enable you to collect and organize your personal data, including photos. Google Pack referrals can only be viewed by publishers and users in Australia, Austria, Brazil, Canada, France, Germany, Hong Kong, India, Italy, Japan, Mexico, Netherlands, South Korea, Spain, Switzerland, Taiwan, United Kingdom, United States. Users who are not located in one of these places will view a Firefox referral button instead. When a Windows user you've referred downloads and installs the Pack, Google will credit your account with up to US$2.00.
Adsense – Google Adsense offers site publishers income by displaying content-related ads on their sites. You earn US$100.00 when a user you've referred to AdSense first earns US$100 within 180 days.
Adwords – Google AdWords enables advertisers to create and display ads on Google.com and the Google content network, including AdSense publisher sites. Advertisers can choose their target audience, set their budget, and see their ad almost instantly. When a user you've referred to AdWords signs up and spends US$100 within 90 days, Google will credit your AdSense account with US$20.00.