Archive of European Integration - Includes independently-produced research materials and official European Community/European Union documents
Council of the European Union (comprised of ministers from member states)
Court of Justice of the European Union
European Council (comprised of the heads of state of the member states along with the President of the EU)
For a complete list of documents by all all EU institutions, consult the EUROPA Official Documents page.
Many public facing publications of the EU can be found free of charge at the EU Bookshop.