The Centre for Longitudinal Study Information and User Support is an ESRC-funded support team for UK academic, statutory and voluntary sector users of the Office for National Statistics (ONS) Longitudinal Study (LS).
The Keep is a centre for archives that opens up access to all the collections of the East Sussex Record Office (ESRO), the Royal Pavilion & Museums Local History Collections and the University of Sussex Special Collections. The Keep provides access to some local, historical census returns and parish registers.
The Northern Ireland Longitudinal Study Research Support Unit (NILS) provides longitudinal data from the Northern Ireland Health Card Registration system to the 1981, 1991, 2001 and 2011 Census returns.
The Google Public Data Explorer aims to make large, public-interest datasets easy to explore, visualize and communicate. Datasets covered are limited but include World Development Indicators (World Bank), Human Development Indicators (UN) and OECD Factbook.
Although not completely authoritative, the easy user interface will take you quickly to informative data. Sources for all facts and figures are cited. Most statistics come from international agencies, such as U.S. Census Bureau and UNESCO.