Rulers.org - An excellent website dedicated to listing all known heads of states of existing and past countries. Also includes lists of religious leaders and leaders of various international organizations.
Regnal Chronologies - A collection of detailed King Lists from every region of the world.
NNDB - A site that gives some brief descriptions of famous people (race, gender, religion/lack thereof, sexuality, etc.) as well as a brief biography of the person in question as well as their family members; it includes everyone from royalty to politicians to musicians to actors. It also has the nifty feature of being able to track connections between people. Should be useful for some, at least.
Emperors of the Sangoku - Rulers of the “Three Kingdoms,” of India, China, and Japan. Includes tables, maps, and chronologies.
The A political biographical dictionary by birth thread - A thread collecting text files with biographies of various famous (mostly political) personalities throughout recorded history. Beware, the thread is a Work In Progress - therefore, some of the earlier posted text files might be outdated versions and some file attachments might be currently removed, replaced or missing.
Most likely names to be given to children in the House of Windsor - A spreadsheet made by doing a bit of statistical analysis, courtesy of Alex Richards.
War Composers - A website dedicated to retelling the stories of the generation of classical composers who fought in World War I.
The Royal Forums - Discussion board on past and present royals and royal families.
Quote Investigator.com - “Exploring the origins of quotations”.