A great place to start. These reference materials link to significant primary and secondary sources for a range of international law topics, and provide definitions to key legal terms.
The Max Planck Encyclopedia of Public International Law (MPEPIL) is a comprehensive online resource containing over 1600 peer-reviewed articles on every aspect of public international law.
Combines terminology and pertinent descriptive information on essentials of international law, including legal terms, prominent decisions, incidents, judicial and literary figures, treaties and conventions and organisations and institutions. References for further research includes cases, treaties, journal articles, and websites. Alphabetically arranged.
(via HeinOnline) Online database that directs researchers to sources, including encyclopaedias, that compare multiple jurisdictions on focused subjects. Includes international law resources.