It's called Legacy of Romulus, and it's coming your way on May 21st. Features include the ability to play as a Romulan and captain one of the empire's fearsome Warbird-class vessels. Cryptic and Perfect World are also tweaking STO's trait system and its UI. And of course there will be plenty of new story and mission content in the offing as well.
[Update]: The expansion will be free according to a Cryptic forum post.