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.

Laws and Treaties: The United States Government and Native American Tribes
Guide to various federal and non-federal government resources.

Library of Congress Treaty Search Page
Library of Congress has a free treaties database going as far back as the 81st Congress (1949-2016).

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

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