The Library of Congress' Congressional Research Service, in cooperation with the U.S. Senate and Government Printing Office (GPO) has made the massive The Constitution of the United States of America Analysis and Interpretation: Annotations of Cases Decided by the Supreme Court of the United States to June 29, 1992 available at the GPO web site. The volume is both searchable and browsable, and contains annotated references to Supreme Court decisions in their constitutional context. It is arranged by article and amendment and is available in both plain-text and Adobe Acrobat (.pdf) formats. Unfortunately, files are also arranged by article and amendment, making for some extremely large file sizes (over 1 megabyte in some cases). Still, this is a small price to pay for an indispensable constitutional law and history reference.