These are useful online or bibliographical resources on various historical statistics, or on data collecting and calculating.
World Perspective - It's a site that basically combines all data about different countries onto one site. Find all kinds of information on more than 100 countries listed in alphabetical order. Hundreds of stats for most countries grouped in 16 comprehensive categories, viewed by bargraph, scattergraph, maps. Cross variables, add parameters and you can even make your own graph for some of them. Errata for the site available here.
Data on the Net - Search or browse a listing of 363 Internet sites of numeric Social Science statistical data, data catalogs, data libraries, social science gateways, addresses and more.
Geospatial and Statistical Data Center
Historical world population figures and other related statistics
Historical population figures thread, courtesy of Thande
Population Statistics.info - Historical demography of all countries, their divisions and towns.
Finding U.S. and international statistics
Year-by-year population statistics
Politic Resources.net - Lots of useful political and politological statistics.
Historical timelines for less famous countries and territories - Very useful chronological overviews of the history of various countries and territories (mainly smaller and more obscure ones).
Gapminder.org - An interactive database of statistics and data from around the world in many major social categories. The graphs allow you to plot say life expectancy versus income per person from the year 1800 onward.
Internet History Sourcebooks Project - Fairly large and comprehensive site. Has links not only to Modern, Medieval and Ancient History but also breaks it down further into African, Jewish, Indian, Islamic, etc. There's even a section of how the various time periods have been depicted in films as well as Traveler's accounts.
NOAA ESRL - Climate and weather data and maps including NCEP reanalysis (covering 1948-2009). Also has a handy (if limited) data mapping tool.
Necrometrics.com - Statistics and bibliography site, maintained by Matthew White. Lists human death tolls (absolute or estimated) of wars, genocides, famines and other atrocities throughout the 20th century.
Historical statistics that surprised you
Searates.org Port Distances - If you want to know how long it takes to get from port A to port B at speed x, and what the distance is.
Calculators On-Line Center - Over 22,000 on-line calculators for every conceivable subject–from calculating dinosaur running speeds to the effects of nuclear explosions to bra sizes to seeing sunrise/moonrise times for any spot on earth to Calendar conversion calculators for nearly every ancient culture.
Online Conversion - Convert between units of measurement from the international system (metric system), the imperial system, customary units and many more. Offered types of measurement include lenghth, area, mass and weight, temperature. All are available in different units of measurement, past and present.
Online Calculators - Similar to the Online Conversion site. Choices offered are a scientific calculator, statistics calculator, “Exctcalc” calculator, a simple calculator, a simple calculator with an added “paper roll” results display, an RPN calculator, abacuses, simple math calculators, and a percentage calculator.
WolframAlpha - The Computational Knowledge Engine.