New York Careshttp://www.nycares.org
New York Cares coordinates volunteers for 500 flexibly scheduled volunteer opportunities every month. Both one day and ongoing projects are available. Options range from reading to a child to painting classrooms. Registration and orientation are required. Searchable.
The Peace Corpshttp://www.peacecorps.gov
Looking for more than a job? Want to make a difference in the world? Why not try the Peace Corp? This is the official site for this US Government program. Check out options, requirements, employment, and apply online.
Volunteer Match is an excellent site to find a great place to volunteer. You can search by location and interest, and even sign up for "virtual" volunteering from the comfort of your computer.