National Household Travel Survey (NHTS)
The NHTS is the authoritative source of national data on the travel behavior of the American public.  The data set allows analysis of daily travel by all modes, including characteristics of the people traveling, their household, and their vehicles.
http://nhts.ornl.gov/

NCHRP 365 -- Travel Estimation Techniques for Urban Planning
In 1998, drawing upon several data sources including the 1990 Census and National Personal Household Travel (now called the NHTS) the Transportation Research Board under the National Cooperative Highway Research Program (NCHRP) released NCHRP Report 365.  this report provides default values for many of the model inputs and evaluation measures used to support transportation travel demand forecasting.
http://ntl.bts.gov/lib/21000/21500/21563/PB99126724.pdf

On the Map -- Longitudinal Employer-Household Dynamics (LEHD)
The LEHD is a program within the U.S. Census Bureau that links employment records with Census demographic data to produce synthetic Origin-Destination flow data at the Census Block level.  The is relatively new and care must be taken when using it.
http://lehd.did.census.gov/led/

Census Transportation Planning Products (CTPP) -- Data Products
Since 1970 the transportation community has been paying for a special set of tables from the decennial censuses that are designed for transportation planning purposes. For a description of the CTPP see http://www.fhwa.dot.gov/ctpp/index.htm

To begin to work your way through the CTPP products go to
http://www.fhwa.dot.gov/ctpp/dataprod.htm

For another look at the available CTPP data products also see
http://trbcensus.com/products/

Integrated Public Use Microdata Series (IPUMS)
 IPUMS is a microdata set of individual person records drawing data from over 150 years of Censuses as well as the new American Community Survey.  Most population data - especially historical census data - are typically available only in aggregated tabular form. The IPUMS is microdata, which means that it provides information about individual persons and households. This makes it possible for researchers to create tabulations tailored to their particular questions.
http://usa.ipums.org/usa/

Employment Securities Data (ES202) now Quarterly Census of Employment and Wages (QCEW)
The QCEW (formerly ES202) is a Bureau of labor Statistics program that publishes a quarterly count of employment and wages reported by employers covering 98 percent of U.S. jobs, available at the county, MSA, state and national levels by industry.
http://www.bls.gov/cew/

List of Private and Federal Employment Data Sources (Circa 2007)
In 2007 the TRB Travel Data users Forum focused on employment data and this annotated list of Private and Federal employment data sources was put together.
http://edthefed.com/data/Employment_Data_Sources.pdf

County-to-County worker flows from 1970, 1980, 1990 and 2000
http://www.bea.gov/regional/reis/jtw/

Metropolitan Travel Survey Archive
The Archive was developed byt he University of Minnesota as a place to store, preserve, and make publicly available, via the internet, travel surveys conducted by metropolitan areas, states and localities.  It contains the data, documentation and results for a variety of regions throughout the country.
http://www.surveyarchive.org/index.html