New York Cares
http://www.newyorkcares.org/ - Arranges a wide range of projects depending on your interests and time commitment.
http://www.volunteermatch.org/ - Identifies opportunities to suit your needs, including virtual volunteering.
http://www.serve.gov/ - Helps people serve America's communities.
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.