This interactive map of the Paris metro/RER systems and I will be best buds while I'm in France.
How cool is this? It's like MapQuest for the Paris Metro and RER system (which is more like a commuter train, because it has fewer stops and goes farther out into the suburbs).
You just enter your beginning and ending metro stops, and it shows you the most efficient way to get there, including the names of the stops where you have to switch, and which direction you need to take on each new line you jump onto. Via this site I discovered that I don't actually have to make two transfers to get to school... I can just hop on the RER instead of the Metro. Hooray!
This is how I get from my apartment to school. Or the Musée d'Orsay. Or the Eiffel Tower. I would also just have to hop over the river from this line to get to the Louvre. Convenient, no?