See also: http://comicbookplus.com/ for many comics that have gone into public domain. Recently used some cartoon images from there for a presentation in lieu of standard clip-art.
There's a bunch of really amazing SF writers mixed into much of the pulp, like Philip K. Dick, Lovecraft, Poe, etc. There seems to be a lack of a large amount of the old serials but a few of them remain.
Weird and annoying: The copyright expired on much of the works, yet some of the archives refuse to display the text on copyright grounds. Take for example "The Variable Man" [1], which Wikipedia [2] claims copyright already expired on due to lack of renewal. So probably a bunch of these issues could be released if they reviewed the records. (Luckily the Gutenberg Project has a copy available [3])
See also: http://comicbookplus.com/ for many comics that have gone into public domain. Recently used some cartoon images from there for a presentation in lieu of standard clip-art.