Immigration to the United States:

http://lcweb2.loc.gov/learn/features/immig/introduction.html - links to various immigrant groups are the pictures on the left column.  Includes a cookbook (click the potluck) link with recipes and stories from people around the U.S.

http://library.thinkquest.org/20619/Intro.html - Learn about several groups who immigrated to the United States (English, Italian, Irish, Chinese, Africans, German, Japanese and Jewish).

http://www.cis.org/ - The Center for Immigration Studies conducts research and provides policy analysis of the economic, social, demographic, fiscal, and other impacts of immigration on the United States.
http://www.whitehouse.gov/cea/cea_immigration_062007.pdf -  Document explaining government’s view of current immigration.

http://fisher.lib.virginia.edu/collections/stats/histcensus/  - Search for historical information from past US Census reports.

http://www.census.gov/main/www/popclock.html  - Use the US Census population clock to learn the current estimates of the population and the demographics.

https://www.cia.gov/library/publications/the-world-factbook/print/us.html - The World Fact Book with current information about the population of the U.S.

Ellis Island:

http://www.ellisisland.org/ - Search for a specific passenger, read stories from immigrants, attempt to pass the citizenship test and more at this site.

http://www.ellisislandimmigrants.org/ - read about the history of Ellis Island, search for an ancestor, access records from other countries, and view the Ellis Island timeline.