A few weeks ago someone pointed me toward this interesting writeup of 50 social sites, it is geared toward sites where businesses should try to build and maintain a presence, but I think that it is applicable for NPO and NGOs too. The article is split into 5 sections:

• Social-Media/Social-Bookmarking Sites
• Professional-Networking Sites
• Niche Social-Media Sites
• General Social-Media Sites
• Job Sites

It includes some of the obvious sites like Digg, and Del.icio.us, LinkedIn, Plaxo and Tribe, Wikipedia and 43Things, and Monster. But there are also some sites that I had not thought of or heard of like Wet Paint for free wikis, SEO TAGG for articles about search engine optimization. The article provides a pretty good primer on what each of these 50 sites specializes in, so it is a pretty good resource to review if you have heard of Furl, or Squidoo but have no idea what they are!