There are many reasons to applaud Paris's system of public transportation. The subway will take you pretty much anywhere with 380 stations on 14 lines. The buses fill in the gaps while also providing you with a front row seat on all the goings on about town. I still get a thrill when the bus goes around Étoile, the traffic circle at the Arc de Triomphe, in part from watching the madness of the cars and motorcycles jockeying for position and in part due to the grandeur of the Arc itself.
The week we arrived, my husband's assistant marched us down to the sales office of the RATP (the authority that runs the subway and buses as well as some of the suburban trains) and got us signed up for a Navigo Intégrale pass. (Well honestly, we were clueless; without her, our first few months would have been much more rocky.) At first, I thought it was just like the SmartTrip proximity card in DC but it turns out that it's so much more. For a flat monthly fee and a one-year subscription, the Navigo Intégrale gives you unlimited rides within the zones of your choice. If you take 20 round trips a month (and honestly, I take A LOT more than that), it pays for itself. Even better, it has a microchip instead of a stripe so you never have to worry about it being demagnetized. I never have to buy tickets (except for the kids) and it gives me a feeling of freedom that's far beyond the cost of the trips.