Changing the world is as simple as changing your homepage

What does your homepage open up to? Google? Use the Toolbar, it's faster. NYTimes? Get it through RSS. Instead, open up your homepage to The Rainforest Site, so that each time it loads you remember to click the button and save "11.4 square feet of endangered rainforest". Could care less about rainforests (and their life savings secrets?), what about The Hunger Site, which allows you to click and provide "1.1 cups of staple food" for the hungry. There's also The Breast Cancer Site ("free mammograms for women in need"), The Child Health Site ("basic but critical health services to...children"), The Literacy Site ("helps children in need discover the joy of books") and The Animal Rescue Site (".6 bowls of food and care to a rescued animal in a shelter or sanctuary")

Pick a site.
Set your homepage.
Click once a day.

:)
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Time commitment? Less than 15 seconds to set your new homepage. Just navigate to the page you want, and then (in Internet Explorer) go to "Tools", "Internet Options" and then click "Use Current" to set the homepage.
Cost? Zero!
Impact? Small (see above descriptions), but it alllll adds up....