Census records at HeritageQuest
One of the largest sources of free scanned census images can be found at HeritageQuest. This repository contains U.S. Censuses from 1790 - 1820 and 1860 - 1930.
Access to HeritageQuest requires log in through a library that is subscribed to the database. To find out if your local library system is subscribed, go to your library's website and search for HeritageQuest. If found, you should be given the option to log in at HeritageQuest with your library card number from home.
The HeritageQuest search engine only matches exact terms, therefore some troubleshooting may be required for abbreviated names and surnames with alternate spellings. The advanced search option is best for this, allowing you to customize your results by county, sex, age, race and birthplace.
Census records and data at FamilySearch
While FamilySearch only provides scanned images for the 1850, 1870, 1900 and 1940 censuses, plenty of information is provided for census years 1860 - 1930 to help you improve your search results at HeritageQuest. I like to have both sites open in separate tabs when searching HeritageQuest for census images.
In most cases, FamilySearch provides a list of each person in the household, gender, age, location and birthplace information. This data can be utilized to customize HeritageQuest census searches when the name seems to be throwing your search results off the right trail.
1940 Census at Ancestry
While the 1940 census isn't the only one available for free at Ancestry, it is the most prominent until FamilySearch has finished integrating the last remaining states into their searchable database.
There is no subscription required to view and save the 1940 Census images to your computer. You will, however, need a free member account. Don't waste your 14 day free trial on this, just sign up for the free account (if you don't have one yet), then go to the 1940 Census search page.
Please feel free to share any other free census resources in the comments below.