Here's a few cool places from Gnomedex that you should know about:
Half the Sky -- Helping people make their lives better
Mental Health Camp -- Erasing stigma and exploring possibilities with social media
Hastac -- People collaborating wiht technology
TweetCloud -- Follow Tweet topics
StarFall -- Help encourage early readers
KidZui -- Kid friendly web browser
Tech Craver -- Using mobile technology to help people
Song Trellis -- Making music, that YOU can make, see this link also
One Bus Away -- Find bus stops near you
Skittlr -- Create a widget with the web sites you want in it
Free app for your Andriod phone, the Tricorder.
National Marrow Donation Program -- Help find matches.
The Girl Effect -- Change the course of history
From the Open Mic portion of Gnomedex. Go check those all out.