Off the top of my head (and in no particular order):

Hildegard von Bingen (medieval church composer)
Clara Schmann (wife of Robert Schumann, and "close friend" of Brahms)
Ruth Crawford-Seeger
Alma Mahler
Amy Beach
Meredith Monk
Jennifer Higdon
Thea Musgrave
Augusta Reed Thomas
Pauline Oliveros
Melinda Wagner
Kaija Saariaho

The last seven of those (Monk through Saariaho) are all still alive
and are pretty well recorded.  Jennifer Higdon and Augusta Reed Thomas
are both "young composers" (in their 40's), and are probably the best
represented on disc.  Thomas is composer-in-residence with the Chicago
Symphony and Higdon teaches at Curtis.  There are plenty more, those are
just the ones I can easily remember.

-Nathan L.
Boston, MA