Westminster Local Economic Assessment 2014 [PDF] - Lots of useful information about Westminster's economy and population, even transport. Some Westminster population figures are also available.
Geoanalyzer from Encyclopedia Britannica - Vital statistics on all countries of the world, allowing comparisons on a range of variables.

Office for National Statistics - Statistics from the government statistical office, including a Neighbourhood Statistics section great looking at statistics for your local area using results from the 2011 Census. Information from the previous 2001 Census is also still available.
Data.gov.uk - A new government statistics site aiming to make them more accessible and therefore to provide greater insight into them.
Health Profiles - Shows the health of people in local authorities across England.
World Bank - Complex site with a wide range of statistical information available, on individual countries and development topics.
UN Statistics Division - United Nations site with a wide range of important data from around the world.
Bank of England statistics - Important British economic statistics including exchange rates, both current and historical from over 40 countries and as far back as 1963.

Currency converters:
- Bank of Canada: Convert from one currency to any of 66 others. Includes historical exchange rates back to 1990.
- OANDA.com: conversion and history for 164 currencies
- XE.com: current and historical rate tables (under "tools" on this site).
- Google Finance: converter and history
Online conversion - All kinds of measures (e.g.: metric / Imperial) with equivalents.
Measuring worth - Comparative tables on money and the purchasing power of the UK pound, in history through to the present.