Office for National Statistics: http://www.statistics.gov.uk/
A portal for the UK government's statistical service, which includes some freely available data and a listing of data sets which can be purchased.

The Market Research Society: http://www.marketresearch.org.uk/
With over 8,000 members in more than 50 countries, The Market Research Society is the world's largest international membership organization for professional researchers and others engaged or interested in market, social and opinion research. The site contains a number of useful links.

The World Association of Opinion and Marketing Research Professionals: http://www.esomar.org/
ESOMAR's mission is to promote the use of Opinion and Marketing Research, for improving decision making in business and society, worldwide.

SecondaryData.com: www.secondarydata.com
A website provided by Decision Analyst, Inc. which offers organized, indexed and annotated links to sources of free economic and marketing research data available on the Web. Includes links to demographics, Internet usage, government figures, international data, and public opinions. There's also a large general reference section of links, and links to tutorials on how to conduct research. Most of the links go to free data.

World Population: http://www.census.gov/ipc/www/idb/
World population data from the US census.

UpMyStreet.com: http://www.upmystreet.com
An interactive site which highlights differences between areas in terms of social structures. Enter a postcode or a name of a town and a page will appear that displays data on that area. Data includes unemployment statistics, house prices, council rates, and local community services.