Courtesy of Thomson Reuters, here is a map of Standard & Poor's sovereign debt ratings by country. The US has just lost its triple A rating and joins China and Spain with a double A plus rating. Notice that Canada belongs to a relatively elite group of countries with a triple A rating.
As for the US downgrade, here are some bright moments from other countries. Canada lost its AAA rating in April 1993 when the Canadian Bond Rating Service downgraded the country's crediting rating from triple-A to AA-plus. The year after, Canadian stocks gained more than 15%. The Tokyo stock market climbed more than 25% in the 12 months after Moody's downgraded Japan in November 1998.
Felix Solomon has an interesting post on soverign debt ratings as well as a link in his article to all S&P sovereign ratings actions since 1975. A very interesting read.