Fitch should be available right here: Sovereign Ratings History
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 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.
UPDATE: That link no longer works. Start page for all S&P ratings is here: http://www.standardandpoors.com/ratings/en/ap/ 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.)
UPDATE: That link no longer works. Start page for all S&P ratings is here: http://www.standardandpoors.com/ratings/en/ap/ 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.