The World Treaty Index began life in 1974 and has been in more or less continuous development since that time, as the output of the database moved from print to various electronic formats. Now it's managed by researchers from the University of Michigan who have given it a new web interface. (See also the explanatory article on Computational Legal Studies.)
The WTI contains only metadata, as it were, about the treaties, and not the texts themselves, which likely can be found in other online databases, such as the United Nations Treaty Collection. Even so, the database is large enough, containing information on nearly all treaties formed during the latter part of the 20th century, which is to say more than 70,000 documents. The Index lets you search by country, laterality (e.g. bilateral, multilateral), date and date range, topic, and keyword in the treaty title. Thus, for example, a search for [Canada] and [bilateral] returns 1277 results (the first of which is a treaty signed in 1946 with the United States on Storage and Loading Facilities at Prince Rupert), each with its associated treaty number and source for text.
As well, results of a search are graphed, letting you see easily, for instance, that Canada's peak period for bilateral treaty making was in the mid 70s:
click image to enlarge
The WTI lets you download the entire database, if you would wish to run it locally, and a CSV file of your search results.
The Electronic World Treaty Index has been updated and relaunched. The features of the new system are described in a new post on VoxPopuLII: http://tinyurl.com/4xnuyfh