This is one of the newer generation of connectors used on small to medium-sized Li-Po batteries.  This connector is easier to solder than the Deans-type connector, and has a slightly higher amperage rating.  It's also easier to plug and unplug, due to the ridges that are molded into the sides of the plastic connector.  Sold in packs of 5 pairs.
 
NOTE: The polarity is marked on these connectors on the back/side of each half, a very tiny + and - sign that is molded into the nylon body of the connector itself. Tin the ends of the connectors first, then insert your wires and add solder until the cavity is filled.