Sovereign Default Under Fiat Currencies
Admittedly, looking at only defaults under fiat currency excludes all the notorious cases before the mid-twentieth century. So I decided to do a little research, and discovered that Standard and Poor's reports 84 sovereign defaults from 1975 to 2002. The following is from their list of States (along with the dates) that defaulted either on their domestic debt, on their foreign debt, or on both. I've excluded cases like Mexico (1982-1990), where the default was not on government securities but on foreign-denominated bank debt, which shortens the list considerably. But some governments, like Argentina's have had more than one default.
Angola (1976), Argentina (1982, 1989-1990, 2002-04), Bolivia (1989-97), Brazil (1986-87, 1990), Congo (1979), Costa Rica (1984-85), Croatia (1993-96), Dominica (2003-04), Dominican Republic (1975-2001), Ecuador (1999-2000), El Salvador (1981-96), Gabon (1999-2004), Ghana (1979, 1982), Guatemala (1989), Ivory Coast (2000-04), Kuwait (1990-91), Madagascar (2002), Moldova (1998, 2002), Mongolia (1997-2000), Myanmar (1984, 1987), Nigeria (1996-88, 1992, 2002), Pakistan (1999), Panama (1987-94), Paraguay (2003-04), Russia (1998-99), Rwanda (1995), Sierra Leone (1997-98), Solomon Islands (1996-2004), Sri Lanka (1996), Sudan (1991), Ukraine (1998-2000), Venezuela (1995-97, 1998), Vietnam (1975), Yugoslavia (1992), Zimbabwe (1975-80).