In order to promote access to research data, many funding agencies require that data produced through funder research be shared with others. Some also require a formal Data Management Plan as part of the funding application. Search for research funders' open access and data archiving policies using SHERPA/JULIET.
This list is not comprehensive - always check with your funding agency for specific and up-to-date requirements.
National Endowment for the Humanities (NEH): Data Management Plans for NEH Office of Digital Humanities (pdf)
National Institutes of Health (NIH): NIH guidelines on data management plans. See also the NIH Data Management and Sharing Frequently Asked Questions.
National Science Foundation (NSF): Dissemination and Sharing of Research Results. See also the NSF's Data Management & Sharing FAQ. Note that some NSF Directorates, Programs, and Divisions have their own requirements.
United States Department of Education (US ED): Institute of Education Sciences (IES) Data Sharing Policy