Bitcoin sounds like something from science fiction: A purely digital currency, created by an anonymous hacker, that operates outside the world's traditional banking systems. The four-year-old currency is very real, though, and it's trading an all-time high, tripling in value in the last two months alone. One bitcoin is was worth about $40 U.S. dollars on Tuesday, and surged on Wednesday to nearly $49. That's up from around $13 in January, and 5 cents in 2010, according to Mt. Gox, the bitcoin market's main exchange.