If you don't own an iPhone (or better said if you're not owned by an iPhone) and hence don't have the handy metro application for iPhone, you can use the online interactive metro-map. (you can use it on any mobile device that lets you brows websites)
Mark the starting station and your end point and the site gives you the best way to go with the time it will take, the cost etc.
like: Very complete
dislike: it's in French only at least i was not able to find a menu where you can change the language.
The main information site of RATP comes in several languages
Si vous n'avez pas de iPhone, et donc vous ne pouvez pas consulter l'application Metro, vous pouvez consulter le plan de metro en ligne, préparer votre itinéraire, voir l'état de trafic etc.
Très complet et facile a utiliser. Et si vous avez un iPhone ou autre téléphone mobile qui permet d'accéder le www, vous pouvez en profiter en route!