While waiting for the 1940 Census indexing projects to be completed, I am using several different websites to determine the Enumeration District for a household and then page through the ED online to find the household of interest.
Steve Morse’s 1 Step Web Pages, at: http://stevemorse.org/ provides this capability with the option to translate the 1930 ED to the possible 1940 ED. Or, search by address for the ED. By clicking on the ED, the Morse website provides links to other websites hosting the 1940 Census images. Ancestry.com has a similar search ability if you know the address the family resided at in 1940.
The National Archive’s at nara.gov also has an address search capability for the 1940 Census. They have the best user interface for doing an address search, and I found their image of the census page to be the best so far. The image appears much crisper than familysearch.org or Ancestry.com. You can go directly to the 1940 Census site at NARA by using the following URL: http://1940census.archives.gov/.