FamilySearch - great for finding free original documents
World Vital Records - I've found lots of published genealogies on this website that have been really helpful.
Chronicling America - a newspaper site that has the actual images. It's kind of hit and miss, but luckily I've had enough hits that I keep going back for more.
Cemeteries and Death Records
Find A Grave
Antiquus Morbus - great for understanding some of the old causes of death. The owner of the website, Rudy, will also help you with deciphering causes of death if you send him a scanned image of the death certificate.
Tri-Counties Genealogy and History - Amazing amounts of information for Chemung County, NY, Bradford County, PA and Tioga County, PA
Old Fulton NY Post Cards - Don't let the title of this website fool you. I did one time in the past, but revisited it after reading about it in Family Tree Magazine. It highlights small town newspapers in New York and I can't believe how many tidbits I found on my husband's family! Mostly social items, but also news articles and birth/death announcements.
Ohio Genealogical Society - My membership gets me access to online databases.