No story about Paris is complete without mentioning Métro or Métropolitain, the underground rapid transit of the city. Famous for its Art Nouveau style ( metro was first opened for the Exposition Universelle in 1900 when art nouveau was à la mode ) the signage is quite something. Unfortunately I dont have much chance to find more signs of Metro, nor enjoy riding this transportation. may be some other time.
For more info about Paris Métro find it here, a brief yet clear overview from wikipedia. If you want to know more, find it here, a nice story with more photos of beautiful metro signs and stations.