NASA is developing a new type of spacecraft engine that combine the performance charachteristics of NERVA type engines and ion rockets.
I don't know how close to actual deployment this engine is but it looks promising to this laygeek.
There's info on the new engine, here and here.
UPDATE: Got permalinks working, by the inconvinient expedient of using a different computer.
UPDATE2: Rand Simberg has graciously responded to my inquiry.
It looks like VASIMIR is neither as mature nor as effective as is being hyped, Oh well, ORION is cooler anyway. :)