Your favorite online collaboration site?

At my company, Pearson, we have a social media collaboration site, and someone recently asked what our favorite online collaboration sites are.  I liked my answer enough to post it here.  What are yours?

I'm liking Global Voices.  To quote from their site, they are  "a community of more than 300 bloggers and translators around the world who work together to bring you reports from blogs and citizen media everywhere, with emphasis on voices that are not ordinarily heard in international mainstream media.....Our international team of volunteer authors and part-time editors are active participants in the blogospheres they write about on Global Voices.....

Global Voices is translated into more than 30 languages by volunteer translators, who have formed the Lingua
 project. ...We also have an outreach project called Rising Voices to help marginalized communities use citizen media to be heard, with an emphasis on the developing world."

http://www.globalvoices.org

And, on a lighter note, the Harry Potter Alliance, working online to turn the energy of Harry Potter fans into fundraising for a multitude of good causes.

"Just as Dumbledore’s Army wakes the world up to Voldemort’s return, works for equal rights of house elves and werewolves, and empowers its members, we:Work with partner NGOs in alerting the world to the dangers of global warming, poverty, and genocide. Work with our partners for equal rights regardless of race, gender, and sexuality. Encourage our members to hone the magic of their creativity in endeavoring to make the world a better place. Join our army to make the world a safer, more magical place, and let your voice be heard!"  They're currently raising money and getting books donated for, among other causes, building a library in Brooklyn.

http://thehpalliance.org/