The columns in the index table on the main page refer to the following editions of Luther's works:
The St. Louis German Edition ("St. L."): Dr. Martin Luthers Saemmtliche Schriften (The Complete Works of Dr. Martin Luther), Dr. Johannes Georg Walch, Ed., Concordia Publishing House, St. Louis. 22 vols., 1885-1910 (Revision of 24 vol. 1740-1753 edition)
The American English Edition ("AE" or "LW"): Luther's Works, Jaroslav Pelikan and Helmut T. Lehmann, Ed., Muhlenberg Press, Philadelphia. 55 vols., 1957-1986
The Weimar Edition ("Weimarer Ausgabe" = "WA"):, D. Martin Luthers Werke: Kritische Gesammtausgabe (Dr. Martin Luther's Works: Critical Collected Edition), Hermann Böhlau, Weimar. Later volumes published by H. Böhlaus Nachfolger. 121 vols., 1883-2009, including:
DB = Die deutsche Bibel (The German Bible), 15 vols.
Br = Briefwechsel (Correspondence), 18 vols.
TR = Tischreden (Table Talk), 6 vols.
Wikipedia article on the Weimar edition - This has an excellent description of the work as a whole as well as a list of the individual volume titles.
www.lutherdansk.dk - A Danish site that not only has a list of the volumes of the Weimar edition and the original Walch edition, but also has links to the works themselves.
Project Wittenberg's Luther section - Twenty or so selected works of Luther's translated into English.
"Beggars All" Blog - A blog entry that has a very good description and partial list of contents of the Walch and the revised Walch ("St. L.") editions of Luther's works.
Concordia Publishing House Prospectus - A detailed description of the contents of the new series of volumes (56-75) being added to the American edition of Luther's Works (LW) by CPH.
therebelgod.com/Luther - Contents by volume (but without page numbers) of the American Edition and Weimar edition.
Back To Luther Blog information on the St. Louis edition - The second part of this post has lots of valuable information on the St. Louis edition, including links to download many of its volumes in PDF format. Also here is a downloadable spreadsheet of Luther's works cross-referenced by the St. Louis edition, the American edition, the Weimar edition, and the Erlangen edition.