Some of the 1 billion UK- and Commonwealth-centric records here, such as censuses, are available elsewhere. But less-familiar sources such as probate registers and trade directories might hold the answers to your ancestral puzzles. One year costs just 49.95 USD.
Locate your kin in this exhaustive place name index of over 280,000 entries. Each lists the historical county and the main administrative areas in which each place lies, both before and after the 1844 Counties Act.
A veritable pot of gold for Irish researchers, this free site features Irish censuses (1901, 1911 and surviving fragments), tithe applotment books, pensioner records, will collections, marriage records and Catholic convert rolls.
This National Records of Scotland site is sure to please with its more than 100 million church registers, censuses, valuation rolls and other documents. Access to wills is free (as is searching generally), but other records are pay per view.