The Digital Comic Museum offers free access to hundreds of pre-1959 comic books, uploaded by users who often offer historical research and commentary alongside high-quality scans.
The site’s moderators and administrators are particularly careful to avoid posting non-public-domain comics (a complicated designation, as described in this forum thread). The resulting archive is devoid of many familiar comic-book characters, like those from Marvel, D.C., or Disney.
On the other hand, because of this restriction, the archive offers an interesting window into the themes of lesser-known comics in the Golden Age—romance, Westerns, combat, crime, supernatural and horror. The covers of the romance comics are great examples of popular art.
Interested in understanding how homefront American culture reflected fighting in World War II and Korea, and the anxieties of the Cold War? The archive is full of titles like "Fighting Yank" (or "Wartime") that trade on true stories of past combat and present-day engagements. Many, like these “Atomic Attack” books from the early 1950s, have a distinctive Cold War flavor, with science-fictional imaginings of futuristic combat. ("See how the war of 1972 will be fought! The war that YOU, yourself, might have to take part in...")
The museum holds some unexpected and forgotten titles, like the Mad Magazine knock-off “Eh.” Here you can see how looking at a comic that wasn't successful enough to have a lasting legacy (and, therefore, a renewed copyright) can be enlightening in and of itself. What subjects did "Eh" cover that Mad might have avoided?
The DCM asks users to register and log in before downloading comic files. Registration is free, and—for now—there’s no limit on the number of titles you can download. You can enter the archive here.
When you're there, make sure you visit the site's ever-growing collection of those notorious 'Pre-Code' Horror comics of the 50s. Also see the Archives and Collections area where artists of note have been given their own individual spotlight.