An Oyster Card? No this is not the secret ticket to an all-you-can-eat buffet, but your ticket onto London's immense transit system. In reality, it is one of the most useful pieces of plastic in London...after your credit card.
The Oyster Card is a smart card, just like those you'll find for transit systems in most big cities. You fill it or top it up with money, which in turn allows you take single trips, buy a multi-day Travelcard, or even a bus pass. The Oyster Card allows you access on a variety of transportation systems, including the Tube, the DLR, overground trains, and even on riverboats. I've found purchasing a travelcard on your Oyster card economical on my past trips to London. It saves you a lot of money over paying single fares, especially if the pound isn't your native currency.
I first discovered the Oyster card on my first trip to London when I wanted to get a Travelcard and expected to a paper pass in return. Instead, the man at the ticket window handed me this little blue plastic card, nestled in its own carrying case. Little did I know that this little card would help me jaunt around London without a care in the world. I didn't have to carry around a flimsy paper ticket, worry about how many trips I took, or waste valuable time buying lots of tickets.
And make sure you save your Oyster Card for your next trip, which will allow you to avoid the lengthy queues at Heathrow Airport. Just register it and top it up online before you go and hop on the train before the rest of those silly tourists.