OPeNDAP, an acronym for "Open-source Project for a Network Data Access Protocol", is a data transport architecture and protocol widely used by earth scientists. The protocol is based on HTTP and the current specification is OPeNDAP 2.0 draft. The protocol is maintained by OPeNDAP.org, a publicly-funded non-profit organization that also provides free reference implementations of OPeNDAP servers and clients.
An OPeNDAP server can serve an arbitrarily large collection of data. Data on the server is often in HDF or NetCDF format, but can be in any format including a user-defined format. Compared to ordinary file transfer protocols (e.g. FTP), a major advantage using OPeNDAP is the ability to retrieve subsets of files, and also the ability to aggregate data from several files in one transfer operation.
OPeNDAP is widely used by governmental agencies such as NASA and NOAA to serve satellite, weather and other observed earth science data.
More information:
http://en.wikipedia.org/wiki/OPeNDAP
