To promote access to research data, many funding agencies require that data produced by funded 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
Browse data sharing requirements by federal agency
Always check with your funding agency for up-to-date requirements.
National Endowment for the Humanities (NEH): Data Management Plans for NEH Office of Digital Humanities (pdf)
See also Data Management Plans From Successful Grant Applications (2011 - 2014) for examples.
National Institutes of Health (NIH): NIH guidelines on data management plans.
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 and solicitations may have provide additional details about special requirements.
United States Department of Education (US ED): Institute of Education Sciences (IES) Data Sharing Policy