This guide contains links to great free reference websites as selected annually by Library Journal and by the Reference and User Services Association of the American Library Association.
The Emerging Technologies Section (ETS - formerly Machine-Assisted Reference Section (MARS)) of the Reference and User Services Association of the American Library Association (RUSA) compiled the list of Best Free Reference Web Sites. They issued a combined index for 1999-2016 [https://www.ala.org/rusa/awards/etsbestindex]. This compilation included the year of review. They published a list in 2017, but have not published one since then.