Ancestry.com: http://www.ancestry.com/
Family Search free website: https://www.familysearch.org/en/
Geni.com: http://www.geni.com/
Legacy.com: http://www.legacy.com/
MyHeritage.com: http://www.myheritage.com/
The Peerage.com: http://www.thepeerage.com/
United Kingdom, Free BMD (Birth, Marriage, and Death) Records: http://www.freebmd.org.uk/
That website I keep getting emails about;
National Inventory of Documentary Sources in the United Kingdom and Ireland: