The implosion and subsequent liquidation of bitcoin exchange Mt. Gox last month played to the worst fears about digital currencies. Confusing to use and volatile as a store of value, bitcoin has also been the frequent target of hackers, thieves, and incompetent businessmen. Despite bitcoin’s appeal to venture capitalists eager to profit from a new financial system, the value for the average person remains elusive. Circle, a highly anticipated new bitcoin startup that has raised $26 million, is hoping to change all that. The company, which opened its service on Friday in a limited release, hopes to be your friendly online bitcoin bank. Its web-based interface lets you deposit US currency and see it converted instantly to bitcoin without any fees. It physically isolates your bitcoin keys in cold-storage vaults and requires multiple "signatures" to bring them online.