The Citizen Services page of the US Embassy in Paris contains a lot of useful information.
You can consult the website of The Embassy of France in the US, especially the Going to France section, that offers a great deal of practical information.
RATP - Paris Metro System
This site also has a metro navigator and tons of information about Paris.
SNCF
The French railroad system. Check out the information about the "Carte 18-27", a discount card for people under 27.
Mairie de Paris (City Hall)
A well-designed site with lots of useful information about daily aspects of life in Paris.
Radio France
Just pick one of the numerous public French radio stations in the middle section menu and click on "Écouter". A great way to practice aural skills!
ISIC
Information about the International Student Identity Card.
