The following resources contain case law (judicial opinions) for both federal and state jurisdictions. Be aware, none of these resources have a robust citator system like that of Westlaw or Lexis when it comes to validating good law. Many use metrics for how often a cased is cited, however, and are useful for research purposes. The physical Law Library has access to both commercial online and print citators, which can be used to check your independent research.
Google Scholar: Case Law by Google
Link above will take you to court selector between federal and state courts. You can also access the case law search via Google Scholar's homepage by selecting the "Case Law" choice instead of the default choice searching "Articles." Search features are basic in comparison to other free options. There is no formal citator but there is a "Cited By" for each case listing which will show you subsequent cases that have cited to that case, but with no additional information about what type of treatment is indicated.
CourtListener by Free Law Project
Much more robust of an engine than Google Scholar. Features advanced searching, more operators, searching within, case summaries, list of authorities, and visualizations if applicable. All cases are hyperlinked to other resources within the site. Contains the same "Cited By" feature as Google and does include any indication of treatment. CourtListener's search engine is enhanced by an system called "CiteGeist" that works by "analyzing your query and combining that with information about the citations between cases."
Caselaw Access Project (CAP) by Harvard Law School
A project by Harvard to make all published U.S. Court decisions available to the public online in a standardized format from their library's digitized collection. The database is updated on a rolling basis at the beginning of each year with new data. Homepage features an interactive map of jurisdictions, standard search, and a breakdown by jurisdiction reporters. The advanced search features include searching by citation, docket number, or author type. Typical "# cases cite to this case," but again, no treatment indicators. Can sometimes include historical trend analysis for how a case has been cited overtime however. Old version of the site can be found at Public Case Access (by Ravel Law).
Findlaw Caselaw by Thomson Reuters
Findlaw combines codes and case law on one overarching page. The search at the top of the linked page will search codes. Findlaw does also provide a portal to search case summaries or full text. On the main page though, you can select from the available jurisdictions and search cases from there either by party, docket, or free text search (but not by citation). Does not feature any sort of "cite by" or citator feature.
Justia Caselaw by Justia
There is no dedicated search just for case law on Justia. You can use the general unspecified search bar and return results of case law, but it will also return results all other content hosted on the site. Otherwise, you can navigate through the jurisdictional offerings to find your sought after court. Some cases will feature summaries provided by Justia. Also does not feature any sort of "cite by" or citator feature.