Voting: 
https://election.lab.ufl.edu/voter-turnout/
https://electionlab.mit.edu/data

Census or Demographic data: 
https://data.census.gov/profile?q=United+States&g=010XX00UShttps://data.census.gov/profile?q=United+States&g=010XX00US
https://www.census.gov/programs-surveys/acs/data.html
https://experience.arcgis.com/experience/13a111e06ad242fba0fb62f25199c7dd/page/Page-1
https://data.un.org/
https://data.worldbank.org/

Comparative:
https://comparativeconstitutionsproject.org/
https://www.comparativeagendas.net/datasets_codebooks

Legislative histories:
https://www.congress.gov/
https://www.ilga.gov/
https://www.senate.mo.gov/LegislativeLibrary/index.html

US Supreme Court:
https://www.supremecourt.gov/
https://scdb.la.psu.edu/
http://scdb.wustl.edu/data.php

Judicial Governance:
https://www.v-dem.net/data/the-v-dem-dataset/
https://worldjusticeproject.org/rule-of-law-index/

Crime statistics:
https://cde.ucr.cjis.gov/LATEST/webapp/#/pages/explorer/crime/crime-trend
https://www.fbi.gov/how-we-can-help-you/more-fbi-services-and-information/ucr

National Crime Victimization Survey:
https://bjs.ojp.gov/data-collection/ncvs

Public opinion on a wide range of topics: 
https://www.pewresearch.org/

Land Use:
https://glorecords.blm.gov/default.aspx