One of the best efforts to date has been Mashable's "This Week, Tweat to Beat Leukemia!", an idea hatched at SXSW. They are Tweeting and using their Facebook audience to raise money, I have posted this on my Facebook page and I'm re-tweeting. Besides the initial messages, there are updates on the funds raised, and lots of comments from all over the world. I initially was hooked by this effort because it is a cause near and dear to my heart, my sister in-law passed away from Leukemia just 2 and a half years ago, but it obviously has touched many more people all over the world because they are almost at their fundraising goal. I can see how all kinds of Charities and other organizations can use this effectively to raise funds and awareness of the work that they are doing - it's a new fundraising model for the new economy, and with fundraising significantly down, this is an amazing tool.
I recently got involved in a campaign for Share our Strength but it was not as effective or as interactive as this effort. Another good story was Magianos restaurants recent drive to build their Twitter community, they offered a chance to win gift certificates to their restaurants for those that started following them on Twitter - not sure how many they where able to add, but very clever tactic, they got me to follow them.