The Modern Language Association has a spectacular online language map showing the languages of the U.S., as well as a data center with searchable language information. For free! 

These tools will be updated soon with 2010 census data. For the moment, they include 2000 census data, 2005 American Community Survey information, and 2009 MLA survey data on college and university students studying foreign (non-English) languages.

Using these online resources, you can do things like determine the number of people who speak a given language in your zip code; look up the languages spoken in your state or county, with numbers of speakers and age breakdown; get a map of all the universities where a certain language is being taught (or at least was recently being taught) around the country; and even look up the numbers of registered students of a given language at the various institutions.

I used the MLA’s website to find a list of most-spoken languages in New York City. Tremendously helpful in this project!