1940 United States Federal Census - Record Group 29, Series T627
The 1940 (Federal) United States Population Schedule Census (T627) became available for free in April 2012. As of that time, there was no index to Census; hopefully in time an index or indices will become available. There is a community project called "U.S. Census Community Project" which is available for organizations. Stephen P. Morse's website has a helpful 1930-1940 E.D. search utility to help you figure out what possible enumeration district that your ancestor possibly lived in 1940.
Before April 2, 2012:
City ancestors -
If your ancestor lived in a large city it would be helpful to know your ancestors address in 1940. Or if you know the 1930 enumeration district number it would help you locate the 1940 enumeration district number. If you do not find your ancestor in that 1940 E.D. number you could look for a 1939/1940/1941 directory which might list your ancestor's address. Then you can use the free on-line 1940 enumeration district maps (T1224), and free 1940 E.D. utility tool to locate the E.D. number. Hopefully your ancestor may not have moved.
Rural ancestors -
If your ancestor lived in a rural location and did not move between 1930 and 1940 and if you know the 1930 enumeration district number that your ancestor lived in 1930 you can use the free 1930-1940 E.D. search utility to locate what the 1940 E.D. number is.
Enumeration District Tools -
Stephen P. Morse's website has a 1930-1940 free E.D. utility available now. You can use the 1930-1940 E.D. search utility to tell you what the 1940 enumeration district is in 1940 based on the 1930 E.D. number. Once you know the 1940 E.D. number and after April 2, 2012 you can browse in that 1940 enumeration district number hopefully you will locate your ancestors. Stephen Morse's free 1940 utility will also tell you what the microfilm roll number is for that 1940 Enumeration District. Then on April 2, 2012 or after can browse the pages in that enumeration district.
The NARA e-store also has a free E.D. microfilm list which tells you which E.D. number is on which microfilm by State, County, and E.D.
On April 2, 2012
With a list of your ancestor who were alive in 1940 by their location, by address, enumeration district number. You can search.