Indexes – Most useful to find a citation by subject matter or chronological date. These entries give you quick facts about each treaty such as signatories and date entered into force.
Flare Index to Treaties (1856 to present)
Multilateral treaties : index and current status & 11th Supp. (1856 - 1994)
World Treaty Index (1900 – 2000)
Multilateral Treaty Calendar (1648-1995) - Organized chronologically but has a subject index. The Table of Contents for this book can be found on Google Books (use control+f to search by subject).
In additional to the full text sources above, these web sites organize treaties by subject.
Environment
Human Rights
Humanitarian Law (Peace and War related agreements)
Intellectual Property
Trade
Full Text Resources - See also the box on Treaties by Subject located below on this page.
For a more complete listing, see EISIL and WorldLII.