The American Public Health Association's Career Mart calls itself the #1 Job Site for Public Health Professionals.
Public Health Jobs, is a hosted by the Association of Schools of Public Health and is a collaborative effort among the CEPH-accredited schools of public health.
USAJOBS is the official job site of the U.S. Federal Government. For example, you could search for announcements in the Department of Health and Human Services. Some scientific positions, can only be found by searching through an agency's individual career pages.
The National Association of County and City Health Officials (NACCHO) maintains a databases of jobs in local health departments.
UVA maintains CAVLINK for students and alumni.
For global opportunities, the World Health Organization maintains a list of openings.