From Daily Kos
CDs-by-County: I think you're all going to love this. Thanks to yet more hard work by jeffmd, we now have a new chart for you: congressional districts by county. Click the link and you'll find a tab for each state laying out which counties are in which congressional district. But there's a lot more. Jeff has also figured out how many people in each county (or part thereof) are in a given district (that's column C), what percentage of each CD is made up by each county or county-part (column D), and finally, what percentage of each county is in a given CD (column E). Since counties are an easily understood and frequently used layer of political organization (for example, party endorsements are often made at the county level), this kind of information is very useful, since it lets you see how much sway a particular county has in a particular congressional district.
(I've changed the example from Arizona to New York)
For instance, if you open the New York tab, you'll see that 198,289 residents of Broome County live in NY-22, and those folks in Broome make up almost 28% of the congressional district's total population. But if you look at column E, you'll see that only 98.85% of Broome is place inside the 22nd—that means it's split among more than one district. Scrolling down (or searching on the word "Broome"), you'll see that there's also a teeny-tiny fraction of the county in NY-19 (just 2,311 people).