You can win $50,000 for the charity of your choice by taking part in America's Giving Challenge, sponsored by Parade Magazine.  In 2008, over 700,000 people participated in the event to raise money for a variety of worthwhile causes.  Here's a little more information from the site:
 

The objective of America's Giving Challenge is to enable passionate inviduals and nonprofit organizations to easily leverage their offline and online social networks to increase awareness about their causes, attract new donors, and encourage people to get and stay involved with causes they care about. By encouraging people to give what they can no-matter how big or small, the Challenge seeks to demonstrate the potential of social networks to bring real people together to make a significant impact.

To investigate further, please visit the link below.