Do jet aircraft have an emergency propeller?
I think you might have heard about the Ram Air Turbine, which is deployed in case of some aircraft in case of loss of main electrical power supply. From A320 Systems briefing:


  In case of total loss of all main generators, the RAT is automatically extended and drives the emergency generator via a hydraulic motor. 


The location of the ram air turbine varies with different aircraft and some of them (like the popular A320) have them near the landing gear. The following image is from @mins' excellent answer to another question.



Location of Ram Air Turbines in different aircraft, image from aviation.stackexchange.com


Bombardier BD700-1A11 Global 5000 (source)
Airbus A320 Flight test (source)
Bombardier CRJ700 (source)
US Navy T-45A (source)
Airbus A330 (source)
Boeing B747-8 (picture by Gregor Schläger, source)
Boeing B777, RAT extension switch (source)