State and County QuickFacts (Census Bureau) - a great place for quick overviews.
County Business Patterns (Census Bureau) - industry information including number of establishments, payroll and number of employees; 1993 onwards; county information is suppressed when an individual business could be identified; search the library catalog for historical data.
LED's Quarterly Workforce Indicators (QWI) (Census Bureau) - for metro areas only; detailed analysis by age, sex, new hires vs. more experienced workers, earnings, etc.; history varies by state's participation.
Statistical Abstract (Census Bureau) - provides historical economic and demographic information
Intergovernmental Google Search
Search, for instance "trade agreement", "human trafficking", etc.