Here are some sites that I've found very useful in my genealogy research. Some of these sites are general, and others are specific to my own family-- i.e., New England, Irish, and Jewish genealogy. 

American Ancestors (New England Historic Genealogical Society)   ($, limited free databases)

Ancestry.com    ($)

FamilySearch  (Free)

Find A Grave  (Free)

Fold3  ($, limited free databases)

 GenealogyBank  ($)

JewishGen  (Free)

Vital Records of Massachusetts to 1849  (Free)