It is unindexed, but it is free and you can download entire microfilm rolls as PDF files--something you cannot do at paid websites.
I find Archive.org difficult to navigate so we will be posting some instructions to get to specific materials on teh site.
To get to a specific roll of 1920 US census microfilm, visit this page to get the roll number:
(you'll need to pick the state you need).
You need the roll number and a word or two from the title--county is a good idea--to perform the search.
Tallapoosa County, Alabama, is roll 43 and 44.
My search for this county is:
14th census population tallapoosa 043
Roll numbers for film on archive.org are usually 3 or 4 digits. "14th census population" pulls up the 1920 census.
My search for:
14th census population tallapoosa 044 did NOT pull up the roll 44 for Tallapoosa County because Tallapoosa is spelled incorrectly.
It might take a little experimentation--but all the 1920 films are there.
14th census of population