Treaties In Force -- S 9.14: --Post 9, Gov. Info Reading Room
Available both on the web and in print, this annual volume lists multilateral and bilateral treaties to which the U.S. is a party. The multilateral (between more than two countries) treaties are listed by broad subject category, while the bilateral treaties are listed by country. Any treaty which is still in effect is included, so it lists some very old treaties. Citations are to the resources listed on this sheet and other treaty compilations.
Multilateral Treaties Deposited with the Secretary-General - UN1/M91/yr. - Post 9, Reading Room
This listing of the status of current multilateral treaties is arranged by broad subject chapters, i.e. Freedom of Information. There is also an index. Current signatories are listed as is the cite to the printed treaty, usually either the UN Treaty Series (UN5/T71/no.) or a General Assembly Official Record Supplement.
HeinOnline
Treaties and Agreements Library: The world's most-comprehensive online collection of image-based U.S. treaties and other international agreements can be found in HeinOnline's Treaties and Agreements Library. This database contains more than 18,000 records of treaties and agreements to which the United States has been a party, and also offers a very powerful indexing tool that is unsurpassed by any other online U.S. treaty database.
World Treaty Index
This 5 volume set indexes not only treaties to which the U.S. is a party but any treaty that the compilers could find. It indexes bilateral treaties between countries, for instance a fishing treaty between Norway and Finland. No other index we have indexes the bilateral treaties between other countries.
Encyclopedia of the United Nations
This work is a good place to begin to research a treaty that you don't know much about. It will occasionally even reprint the entire treaty.