An example of such an enantiomer is the sedative thalidomide, which was sold in a number of countries around the world from 1957 until 1961.  It was withdrawn from the market when it was found to cause birth defects.  One enantiomer caused the desirable sedative effects, while the other, unavoidably present in equal quantities, caused birth defects.
The herbicide mecoprop is a racemic mixture, with the (R)-(+)-enantiomer ("Mecoprop-P", "Duplosan KV") possessing the herbicidal activity.
Another example is  the antidepressant drugs escitalopram and citalopram.  Citalopram is a racemate [1:1 mixture of (S)-citalopram and (R)-citalopram]; escitalopram [(S)-citalopram] is a pure enantiomer.  The dosages for escitalopram are typically 1/2 of those for citalopram. Here, (S)-citalopram is called a chiral switch of Citalopram.