The Nature Conservancy has been using Flickr for their annual digital photography contest for the last 3 years, with some impressive results. The organization realized that a lot of their supporters were also photographers of nature. Meeting their supporters where a lot of them already were online, on Flickr, they were able to tap into the populatrity of the site.
TNC now has over 10,000 members of their Flickr group, one of the largest to date. Thousands of photos have been submitted and hundreds of new e-news subscribers have been gained with every annual contest.
Beth Kanter has a great interview with Nature Conservancy staff members Evan Parker and Susan Citro about how the organization is using Flickr, click here to read it.