Astronomy Virtual Observatories
"A Virtual observatory (VO) is a collection of interoperating data archives and software tools which utilize the internet to form a scientific research environment in which astronomical research programs can be conducted. In much the same way as a real observatory consists of telescopes, each with a collection of unique astronomical instruments, the VO consists of a collection of data centres each with unique collections of astronomical data, software systems and processing capabilities. The main goal is to allow transparent and distributed access to data available worldwide. This allows scientists to discover, access, analyze, and combine nature and lab data from heterogeneous data collections in a user-friendly manner. The IVOA (International Virtual Observatory Alliance) is a standards body created by the VO projects to develop and agree the vital interoperability standards upon which the VO implementations are constructed."
The VO concept is becoming spread around the world and there are a number of recommendations already set by IVOA, regarding vocabulary, metadata and space-time coordinate definitions, among many other necessary characteristics required to implement a VO service. Some examples can be found in the list of 28 publications released on the ArXiv.org/astro-ph website in 2011.
The International Virtual Observatory Alliance
"The International Virtual Observatory Alliance (IVOA) was formed in June 2002 with a mission to "facilitate the international coordination and collaboration necessary for the development and deployment of the tools, systems and organizational structures necessary to enable the international utilization of astronomical archives as an integrated and interoperating virtual observatory." The IVOA now comprises 20 VO programs from Argentina, Armenia, Australia, Brazil, Canada, China, Europe, France, Germany, Hungary, India, Italy, Japan, Russia, Spain, the United Kingdom, Ukraine, and the United States and inter-governmental organizations (ESA and ESO). Membership is open to other national and international programs"
NASA Skyview Virtual Telescope
"SkyView is a Virtual Observatory on the Net generating images of any part of the sky at wavelengths in all regimes from Radio to Gamma-Ray."
The Virtual Observatory
The Virtual Observatory (VO) embodies the concept of large scale electronic integration of astronomy data, tools, and services on a global scale in a manner that provides easy access by individuals around the world. This new environment of interoperability will facilitate astronomy research with a speed, efficiency, and effectiveness not previously possible, and it will be available to all researchers, independent of their affiliation or access to observing facilities. The VO will also provide a unique and powerful resource for initiatives in education and public outreach.
The US Virtual Astronomical Observatory (VAO) is the VO effort based in the US, and it is one of many VO projects currently underway worldwide. The primary emphasis of the VAO is to provide new scientific research capabilities to the astronomy community. Thus an essential component of the VAO activity is obtaining input from US astronomers about the research tools that are most urgently needed in their work, and this information will guide the development efforts of the VAO. Examples of current VAO projects are given on these web pages, and the VAO will maintain a continuing effort to be informed of current astronomy needs and interests. 
List of Virtual Observatory Websites
|United States of America||http://www.us-vo.org/|