Countries Travel Advisories Websites:
Australia's Department of Foreign Affairs and Trade
Canada's Department of Foreign Affairs and International Trade
New Zealand's Ministry of Foreign Affairs and Trade
United Kingdom's Foreign & Commonwealth Office
United States' Department of State
The Commonwealth countries do a good job of providing warnings. The first information displayed, usually in a color-coded system, gives the general advisory and any related important information. New Zealand even provides links from the other countries' travel warnings pages.
The United States' State Department page is horrible, though. One has to scroll through a CIA World Factbook-style introduction and then sort through other general information to get to the advisories. Looking up Iran one will have to read past "Iran is an Islamic Republic" to get to any useful information.