Winkdex.com is powered by Math-based Asset Index LLC, which is also operated by the Winklevoss twins. According to the site:
Winkdex is calculated by blending the trading prices in U.S. dollars for the top three (by volume) qualified Bitcoin Exchanges* during the previous two hour period using a volume-weighted exponential moving average. Constituent Bitcoin Exchanges of Winkdex are selected by Math-Based Asset Index, LLC and are electronic trading platforms on which users may buy or sell bitcoins with other users in exchange for U.S. dollars.
The site’s “Formula” page also offers a few insights into the index’s methodology:
The value of the Winkdex (including a daily spot price) is determined using Bitcoin Exchange transaction data on a moving average basis over a trailing two hour period. The proprietary Winkdex formula not only utilizes the most recent Bitcoin Exchange transactions, but also volume weights those transactions on a proportional basis. In order to reflect the latest in pricing information, the most recent transactions are weighted exponentially greater than earlier transactions in the two hour period.
Currently labeled as being in beta, the site offers a sign-up form for API access. As of this writing, the index valued Bitcoin at $578.17.