Microsoft's charitable effort involves each of their hotmail account subscribers.
It's easy. Once you sign up for their "i'm intitiative" each of your e-mails make a contribution to your chosen charity and every one of your e-mails has an invitation to the receiver to sign up. Now the hitch is you have to use hotmail. Will this drive more people to sign up for hotmail accounts? I don't know, but for those that use it, it's an easy way to feel good about just e-mailing and "spreading the word". You get to pick from several causes they support. I picked Unicef, one of my favorites.
Here's Microsoft's description about their program.
"The i’m Initiative from Microsoft® makes helping social causes easy. Every time you use Windows Live™ Messenger or Windows Live Hotmail®, our free webmail service, we'll share a portion of our advertising revenue with the cause you choose from among our partner social cause organizations. Exactly how much will we share with our partner causes? Well, that's up to you and the growing masses of i’m Initiative users. The more messages you send, the more we give.
Now the small things you do every day, like e-mailing and IMing, can make a big difference."
"i’m is about making a difference. Not in a huge expensive way, not in a time-consuming way. But in a simple, effective way."
I couldn't have said it better