West Germany annexed East Germany in October 1990 after the fall of the Berlin Wall and a period of negotiations between officials of both countries.

Yemen Arab Republic and South Yemen merged in 1990, if that counts. There are far, far more examples of countries splitting up though.

There are other examples of annexations like Iraq and Kuwait in 1990 as well, but part of the problem with this type of question is international recognition. Regardless, that particular situation reversed itself in the first Gulf War.