An ever-growing treasure trove of magazine cover and advertising art from the Golden Age of American illustration. Check out wonderful covers from Theatre Magazine, Adventure Magazine, the Argosy, Photoplay, and Black Mask. Here's a scary cover from Laughter magazine, a strange and beautiful Life cover from 1887, and a copy of The Liberator that I dearly wish I could flip through. See also collections of great old ads for soap, cigarettes and books, among others. The intro page is here.
Galactic Central is the mother lode of magazine archives, with publishing information and cover art, including a prodigious pile of pulp magazines.
Future Magazine covers: A bit self-aggrandizing in a subtile way. However I found the future mag covers engaging enough in a "lite" sort of way. (click on: "See covers from the future" option)(via)
a few thousand science fiction covers is a flash presentation of thousands of SF magazine covers from the fifties through today, arranged chronologically from left to right, and grouped by color from top to bottom.