Yes, and they're also called "cyclocross" or "cross" levers after the racing that popularized them. They're essentially spliced into the middle of the existing brake cables and can be easily retrofitted onto any drop bar bike. I think I've seen them for around $40 for a nice pair and probably cheaper on the net.

EDIT: Here's the link to explain all...http://sheldonbrown.com/gloss_i-k.html#interrupter