I'm a visual person. I spend a lot of time browsing art and illustration forums and blogs, and images I've found on them have given life to campaign ideas I already had and inspired new ones. One of the first things I imagine when I think about a campaign setting is "What does it look like?"
Which is why there are few blogs I follow as closely as Golden Age Comic Book Stories. For the last couple years, "Mr. Door Tree" has posted good-sized, high-quality scans of great art almost every day. His archives are a treasure trove of inspiration. Looking for romantic high fantasy? N.C. Wyeth's King Arthur and The White Company illustrations are a good place to start. Looking for something a little more sword and sorcery? Michael J. Kaluta's illustrations of Robert E. Howard's Swords of Shahrazar are one place to look, and you can't go wrong with a gallery of classic Weird Tales covers or Virgil Finlay's interior illustrations: 1936 -- gallery 1 and gallery 2; 1937 -- gallery 3), either. Maybe you're interested in space opera and science fiction? Buck Rogers and Flash Gordon are here, and they're not alone.
There's so much more. The art ranges from pulp art to Disney to Harvey Kurzman to Charles Addams. When I first stumbled across the blog over a year ago, I spent hours in the archives. Now... well, I'd probaby lose most of a day's work.
It's always hard when posting a link to know whether everyone else already knows about it, or whether people are hearing about it for the first time. Golden Age Comic Book Stories is a much higher traffice blog than mine, but if even one person is hearing about it here for the first time, it's worth my time to link to it.