Data that is either openly-licensed, or in the public domain, and stored in online repositories where the datasets are free to access and download.

Open, freely-usable data makes activities such as 'Open Science' much easier to carry out.  For example, one researcher's data could become useful to a different researcher years later, when analyzed in a different way, or perhaps in aggregate with other studies' data.

Resources that will help you learn more about and work with open data:

• Open Data Handbook

  This guide discusses the legal, social and technical aspects of open data. It can be used by anyone but is especially designed for those seeking to open up data. It discusses why to go open, what open is, and the how to 'open' data.
• Open Data Commons

  Legal tools to assist authors with publishing and using open data.
• The Panton Principles for Open Data in Science

  A set of recommendations on how best to make published data from scientific studies available for re-use.

Licenses for data have been found in various places:

• Embedded in the metadata for the data
• Communicated by watermarks or notices within the data
• Specified on the landing page for the dataset
• Specified on a repository website
• Detailed in a ReadMe file released with the dataset

Multidisciplinary:

figshare

• A multidisciplinary repository where researchers can share data in a citable, sharable, and discoverable manner

ICPSR: Deposit Data

• Multidisciplinary repository designed to provide long-term data storage, including replication datasets tied to published articles

Open Access Directory: Data Repositories

• Looking for datasets for your discipline? OAD repositories and databases for open data, organized by subject

Registry of Research Data Repositories (re3data)

• A global registry of research data repositories that covers multiple academic disciplines. It promotes a culture of sharing, increased access, and better visibility of research data

Open Knowledge International Projects

• Open source data management solutions to facilitate metadata assignment, text and data mining, track open government data, civil data applications

STEM:

Biosharing

• Directory of life sciences databases and reporting standards

DataONE

• A distributed network of data centers, science networks and organizations that provides open, persistent, and secure access to well-described data

Dryad

• An international data repository underlying peer-reviewed articles in the basic and applied biosciences. Owned by a consortium of journals that collaboratively promote data archiving and ensure sustainability

knb

• The Knowledge Network for Biocomplexity (KNB): An international repository for ecological and environmental data

Nature: Recommended Data Repositories

• Nature's Scientific Data requires authors to release datasets, but Nature does not host data. These data repositories meet Nature's requirements for data access, preservation, and stability