Many statistical packages require specific formats for data input and will produce only a limited set of output formats. Please check the help files for the software you are using to ensure that you will be able to get data in the format you need.
ICPSR/ Interuniversity Consortium for Political and Social Research
Social science data sets are available for downloading; coverage includes but is not limited to the U.S. Census, crime and criminal justice, foreign and national opinion polls, health, consumer surveys, and historical data sets. Primary data formats include “three plain text formats (column-delimited ASCII, comma-delimited ASCII, and tab-delimited ASCII), two SAS formats (SAS XPORT and CPORT files), two SPSS formats (SPSS SAV and portable files), and the single Stata data format.”
"The home of the U.S. Government's open data." Includes data sets from Federal and State agencies as well as other organizations. Topic categories include agriculture, business, climate, consumer, ecosystems, education, energy, finance, health, local government, manufacturing, ocean, public safety, science & research. File formats vary for different data sets, but among those available may be Excel, CSV, txt(csv), xls.