The 1940 U.S. Federal Census was released this morning , April 2nd, at 9:00 a.m.
Ancestry.com already has several states loaded. You can browse those images on their site or at Ancestry Library Edition.
Archives.com also gives everyone free access.
Read a fun article by genealogist Megan Smolenyak about Who We're Searching For and Why in the 1940 Census.
Here are a few general links to 1940 Census resources.