The US Department of Housing and Urban Development provides crosswalk data sets that map zip codes to Census tracts, counties, etc. and vice versa. Unfortunately these data only go back to 2010, and use different definitions (2000 Census vs 2010 Census) after a certain point, so I may have to convert these Census divisions anyway (as this question talks about).
Are there any freely available datasets that contain zip code to census tract and/or county mappings (or vice versa) going back farther than 2010? I've read of proprietary datasets that do this in journal articles, but I haven't found anything open and free.