Nonprofit organization classifieds
Good listings of social service volunteer sites, as well as information on jobs
The Catholic Network of Volunteer Services
Might provide some good leads. Obviously most if not all the leads are with faith-based organizations.
Nonprofit Career Center
An alpha list of non-prof organizations, plus some pretty cool job listings, mainly from Mid-Atlantic region, a lot in DC
Human Services Career Network
Your source for Human Service Jobs, Social Service Jobs, Social Work Jobs, Non-Profit Jobs, Mental Health Jobs, and Behavioral Health Jobs.
Most jobs appear to be in large cities, or West/Midwest, but it's nonetheless an encouraging site.
Find Child Care, Tutoring, Pet Care, and Senior Care Jobs and Providers
Jobs in social services and political/social advocacy