The basic organization of primary (or government created) legal documents on the web is generally by jurisdiction, and then by sub units within the jurisdiction. The depth of government sites will vary. They are logically broken down by branch: executive, judicial, and legislative. Typically, they include various types of public documents normally generated in the course of business. The executive branch will include executive statements and reports, agency produced documents, transcripts of agency proceedings, agency rules and determinations, administrative hearing decisions, and other miscellaneous documents. Court sites typically have opinions, court rules, administrative documents, and statistical reports of court business. Legislative sites typically have bills, amendments to bills, public laws, transcripts of legislative sessions, legislative hearings and reports if they exist, calendars, and a legislation tracking system.
One alternative to finding older government materials that likely exist but are not available on the Internet is to contact the state archive. That role is normally filled by the official state library. Older federal materials may be available through the National Archives and Records Administration (NARA) or the Library of Congress. It would be best to speak with a reference librarian at a law library to find the best source and contact information depending on the needed materials. Older municipal documents, for example, can present quite a challenge. The city clerk’s office is a likely source for that type of material.
Materials that are unavailable in free legal research sites include standard legal reporters such as the National Reporter System (at least those published after 1923 and still in copyright). Current treatises, handbooks, or other long-form legal commentary do not appear on free research sites in any significant quantity, if at all. There is no equivalent, for example, to American Law Reports or AmJur2d. Materials related to legal research such as non-law journals, magazines, newspapers, and many scholarly journal archives are mostly relegated to pay-for-access sites, though there is trend for law reviews to place current issues on free sites. Citators do not exist in a form that duplicates the utility of Shepards Citations (Lexis) or KeyCite (Westlaw), online or in print. There is one feature in Google Scholar Legal Opinions and Journals that offers a rudimentary citator at best for cases retrieved through that site.
 An exception is the Social Science Research Network (SSRN), available at http://www.ssrn.com/. Law faculty, among others, place not only published material, but pre-publication scholarship on the site. It’s searchable by keyword and indexed by Google Scholar.