These are sources for data on multiple countries and multiple industries.
Most industries and countries have specialized sources that you may
want to seek out after you have focused on a particular policy.
APEC Tariff Database Online
CIA, World Fact Book Online
Economist Intelligence Unit, Country Studies, Business School Library and Online*
Export.gov Online
Food and Agriculture Organization of the UN, FAOSTAT Online
Global Trade Alert Online
IMF, International Financial Statistics, Foster Library and Online* with links also to Direction of Trade (DOT) and Balance of Payments (BOP).
IMF, World Economic Outlook April 2012 (including statistical appendix) Online
NBER, NBER Data Online
OECD, OECD Statistics Online
Trade Competitiveness Map, International Trade Centre, Online
United Nations Data Online
United World, country reports Online
World Bank, Data and Maps Online
World Bank, Temporary Trade Barriers Database Online
World Bank, World Development Indicators 2009, Online*
World Bank, World Tables, Graduate Library (most recent is 1994)
World Trade Organization, Trade Policy Review (selected countries) Online
World Trade Organization, Trade Statistics Online

*Available only through U of M subscription, with your UM uniqname and password.