HazMap, an awesome site developed by the National Institutes of Health, was listed in the Wall Street Journal's annual list of top new (or improved) health sites. HazMap includes a multitude of chemical profiles of agents that pose a potential occupational risk. The site also includes profiles of high risk jobs.
According to the Journal's article, "The site recently added 180 new chemical profiles, and now covers more than 2,000 chemical agents and 225 occupational diseases. Consumers can search by symptoms or diseases". Other health sites that received mention include: Consumermedsafety.org, Why Not the Best, Everyday Health, and HealthCentral. Among them, only HazMap seems to focus on occupational health issues. The most helpful aspect of the site to me is this form tool that helps to identify occupational diseases by job and symptoms.