The Avalon Project at Yale Law School
This collections bills itself as "Documents in Law, History and Diplomacy". While some of the documents are purely of historical interest, many have current legal standing in international affairs. This is the place to go to find the text of the Dayton Peace Accords, the peace agreement that ended the Bosnian war; the Hay-Bunau-Varilla Treaty, which gave the US a chunk of Panama to build a canal; the Good Friday Agreement, which has brought some peace to Northen Ireland; and a wealth of many other documents.
dead link: http://www.yale.edu/lawweb/avalon/avalon.htm