Paris street signs are not always easy to see, as they're posted on the side of buildings, but they're usually wonderfully informative. Not sure where Rue Monge got its name? It's right there on the sign, noting that Monge was a mathematician and one of the founders of the Ecole Polytechnique. Not sure when Descartes lived? The street sign notes that he was born in 1596 and died in 1650. Some signs are more enigmatic.
Rue des Mauvais Garcons (Street of the Bad Boys), in the Marais, has had that name since the 15th century but no one can tell you why. (Too bad I don't have a companion picture of the rue des Bon Enfants in the 1st arrondissement!)
Literally, the street of the cat who fishes. This street is perpendicular to one of the quais that runs along the Seine and probably tooks its name from a tavern but no one really knows anything about that darn cat.
Oh and while I'm on the topic of street signs, here's a tip for tourists. There's really no concept of a city block in Paris. If you have to ask directions, you'll be told to walk to the next street or walk past two streets. In addition, the numbers on one side of the street don't necessarily correspond with those on the other. There are, in fact, even and odd sides, but one number may be in the 80s while the other will be in the 90s.