"Government information by topic and agency,” for federal, state, and local government data. The best place to start looking for a topic if you're not sure which agency/agencies would be involved, because it will bring back results from all of them! Everything else included in this list will generally also be findable via USA.gov.
More subject-specific portals to government info
Data.gov and Science.gov provide access to information that can also be found via USA.gov, but they're better when you know you're looking for data or STEM research.