**Fitch** should be available right here: *[Sovereign Ratings History][1]*

With **Moody's** it's not so easy, I don't know if there's a complete source available free of charge. But *[Sovereign Default and Recovery Rates, 1983-2007][2]* has some data in the appendix III, though not so up to date and in a not that convenient format.

Same goes about **S&P**, *[Sovereign Ratings History Since 1975][3]*.

UPDATE: That link no longer works. Start page for all S&P ratings is here: [http://www.standardandpoors.com/ratings/en/ap/][4]
You can see latest ratings only. Clicking a country name requires a (free) login, but still only shows the latest data (though it does then have links to methodology, background articles, etc.)

Maybe someone will be able to come up with more edible formats though.


  [1]: http://www.fitchratings.com/web_content/ratings/sovereign_ratings_history.xls
  [2]: http://v2.moodys.com/cust/content/content.ashx?source=StaticContent/Free%20Pages/Credit%20Policy%20Research/documents/current/2007100000482445.pdf
  [3]: http://www2.standardandpoors.com/spf/pdf/fixedincome/KR_sovereign_SovRatingsHistorySince1975.pdf
  [4]: http://www.standardandpoors.com/ratings/en/ap/