To tie in with the Roayl Wedding, Ancestry.co.uk (one of the largest family tree tracing sites) is making the UK marriage indexes free for a short time.
They have more than 96.4 million records of marriages from 1837 to 2005, and The Royal Collection, detailing half a million people born into or descended from royalty, the peerage, nobility and the landed gentry free to access from the 21st to the 30th April 2011.
Worth a look.