International Treaties (GlobaLex)
Also see the portion of the ASIL research guide on Public International Law which discusses treaties (Section III.A., pages 9 to 16)
Some excellent online treaty research guides are available from various law school libraries with large international collections: e.g., Boston University, Columbia, Georgetown, N.Y.U. The Hein Online Treaties and Agreements Library links to some of them. Use search terms such as <treaty research>
For detail about U.S. Treaty Practice, see:
Robert E. Dalton, Chapter 6, "National Treaty Law and Practice: United States,"National Treaty Law and Practice, eds. Leigh et al (1999).