Wikipedia
IAPWS
The International Association for the Properties of Water and Steam (IAPWS) is an international non-profit association of national organizations concerned with the properties of water and steam, particularly thermophysical properties and other aspects of high-temperature steam, water and aqueous mixtures that are relevant to thermal power cycles and other industrial applications.
The organization publishes a range of 'releases.' Specifically, these relate to the thermal and expansion properties of steam.
Both free software and commercial software implementations of the IAPWS correlations are available.
See http://www.iapws.org/ for more details on the IAPWS. Their release documents are freely available from the site.