Although I was driving to work in Elbonia, I now have the (dis)plpeasure to ride the Metro everyday to work.
For january, I had bought what is called a 'Carte Orange', one month pass valid for certain part of Paris underground network.
However, I soon discovered that a all-year cheaper pass was available. Called Navigo, it has the aditionnal perk to be a contactless pass. You don't have to take the ticket out of your wallet - you just pass your wallet/card holder quickly over the card reader logo.
I have although learned that London Underground now offers a similar pass (Oyster card).
Are any other network using such a pass system ? And a crazy idea of mine : do you think it would be possible for me to use my Navigo pass whenever I am in London, a bit like I would use my cellphone in roaming...