"Doubt" is another one; borrowed from old French doute in the 13th century, with the "b" added by scribes in the 14th century.

There's a bit in Shakespeare's _Love's Labour's Lost_ where the pedant Holofernes is ranting about how the "b" should be pronounced in those two words, but as you say, it was always silent.

EDIT: Turns out "salmon" and "solder" were originally "saumon" and "soudor", with the "l" added in the 18th century (agsin to reflect the Latin spelling)