United States of America

Despite bring by far the richest country in the world, the USA suffers from a number of social problems that you can help with:

New York Cares

http://www.newyorkcares.org/ - Arranges a wide range of projects depending on your interests and time commitment.

Volunteer Match

http://www.volunteermatch.org/ - Identifies opportunities to suit your needs, including virtual volunteering.

Serve.Gov

http://www.serve.gov/ - Helps people serve America's communities.

Craigslist

www.craigslist.org - Search for volunteer opportunities (and anything else!) in the area you're interested in.

The Salvation Army

http://www.salvationarmyuse.org/ - Helps rebuild lives, offers comfort and support, and works with local communities.