The Federal Government's official repository for authenticated digital documents. Not only a good source for accessing the US Code, but also various Congressional documents and administrative legal authorities.
While it can serve as a very helpful introduction to important legal concepts for the lay reader (or to an attorney looking for a free secondary source on an area outside their expertise), Justia also provides free and extensive access to primary authorities. The feature that is likely most exciting for practicing attorneys is the large collection of free legal forms.
Not only provides free and up-to-date access to Federal authorities, but also hosts the Wex legal encyclopedia & dictionary, an extensive secondary source authored by volunteer experts on American law.
The Illinois General Assembly's web site not only provides access to the current version of the Compiled Statutes, but also the public acts, pending legislation, and chamber journals & transcripts dating back to 1971. It is an invaluable resource for researching legislative history without paid databases.
While most known for providing quick access to a massive body of scholarly articles, Google Scholar also includes every reported, precedential case. Select Case Law and select your jurisdictions before searching, or enter a direct citation. Cases are paginated according to the National Reporter System. Click on How Cited when reading a case to access a rough equivalent of a citator and find cases that cite the on you're reading. However, be aware that this does not inform you if a case has been overruled or questioned by a higher court!