Setting: do you need details about the physical location of your story (what’s it look, smell, sound, feel like?) What about the time period (does it take place in the past, or future?)?
ProQuest Historical Newspapers can give you a contemporary point of view on your topic, happening, or character.
Point Of View/Dialogue: if you’re using first person, do you need to research your character’s voice/dialect/vocabulary (where is your character from? What’s their education level? Etc.) For all POVs, how do your characters speak?
ARTstor is a database of high-quality images of art, architecture, and photographs.
Character: do you need to know about a particular profession (is your character a doctor, policeman, mad scientist)? Has your character experienced something you need to research (alien abduction, a tour of duty in Iraq, or childbirth for instance)?
Plot: do you need to research some situation integral to your plot (is your character getting divorced? Married? Released from prison? Arrested for murder? Etc.)?