Avalon Project at Yale Law School
Full text of about 30 treaties between the US and Native Americans dating from 1778 to 1868.

Indian Affairs: Laws and Treaties
Edited by Charles J. Kappler. This digital collection contains all seven volumes.

Library of Congress Treaty Search Page
Library of Congress THOMAS site has a free treaties database going as far back as the 90th Congress (1967-1968).

Multilaterals Project
Well organized and indexed site containing texts of international multilateral agreements.

Researching U.S. Treaties and Agreements
LLRX thorough guide to accessing and researching US treaties

Treaty Affairs
Official Department of State site includes current Treaties and Other International Acts Series (TIAS), and the annual summary, Treaties in Force (TIF).