http://www.data.gov/
这个网站汇集了很多学科、不同政府部门的有关数值数据，可以检索、下载等。如果对美国的科学数据感兴趣的，可以参考一下这个网站。

About
The purpose of Data.gov is to increase public access to high value, machine readable datasets generated by the Executive Branch of the Federal Government.

As a priority Open Government Initiative for President Obama's administration, Data.gov increases the ability of the public to easily find, download, and use datasets that are generated and held by the Federal Government. Data.gov provides descriptions of the Federal datasets (metadata), information about how to access the datasets, and tools that leverage government datasets. The data catalogs will continue to grow as datasets are added. Federal, Executive Branch data are included in the first version of Data.gov.

Participatory Democracy
Public participation and collaboration will be one of the keys to the success of Data.gov. Data.gov enables the public to participate in government by providing downloadable Federal datasets to build applications, conduct analyses, and perform research. Data.gov will continue to improve based on feedback, comments, and recommendations from the public and therefore we encourage individuals to suggest datasets they'd like to see, rate and comment on current datasets, and suggest ways to improve the site.

Goal
A primary goal of Data.gov is to improve access to Federal data and expand creative use of those data beyond the walls of government by encouraging innovative ideas (e.g., web applications). Data.gov strives to make government more transparent and is committed to creating an unprecedented level of openness in Government. The openness derived from Data.gov will strengthen our Nation's democracy and promote efficiency and effectiveness in Government.

How do I use Data.gov?
Data.gov includes searchable catalogs that provide access to "raw" datasets and various tools. In the "raw" data catalog, you may access data in XML, Text/CSV, KML/KMZ, Feeds, XLS, or ESRI Shapefile formats. The catalog of tools links you to sites that include data mining and extraction tools and widgets. Datasets and tools available on Data.gov are searchable by category, agency, keyword, and/or data format. Once in the catalog, click on the "name" (i.e, the name of the dataset or tool of interest) and you will be taken to a page with more details and metadata on that specific dataset or tool.