For six weeks in London, I got an Oyster card. Now that I'm traveling into Boston on a regular basis, I got a CharlieCard. You can buy the regular CharlieTicket, but it's cheaper if you get the plastic CharlieCard and so much more fun to have a magnetic reader card for the T.
Fun fact- they picked the name for the card from the "Charlie on the MTA" song. Apparently, they considered calling it "The Fare Cod," cod being a pointer to a common local fish and also how someone with a Boston accent might say card. But someone clearly pointed out how silly that was.