The United States Constitution is available on many websites, including versions with analysis and case annotations.
Constitution of the United States Files are available in ASCII text and PDF format, courtesy of GPO Access. Includes Other House and Senate Document editions and Pocket Editions of the Declaration of Independence with unratified amendments.
U.S. Constitution with Annotations of Cases (FindLaw) Congressional Research Service (CRS) document is indexed by article and amendment and includes analysis with citations and footnotes. Links to cited Supreme Court cases.