To access the free records, visit www.ancestry.com/cs/free-access. According to that page, “*Access to the records in the featured collections will be free until February 8, 2015 at 11:59 p.m. ET. After the free access period ends, you will only be able to view the records in the featured collections using an Ancestry.com World Explorer paid membership. To see a full list of the records in the featured collections please click here.”
The list is too substantial to include here, but it includes the England and Scotland census collection, civil registration indexes for Ireland (1864-1958), civil registration indexes for England & Wales (1837-2005), select births and christenings, select marriages, and select deaths and burials.