The Comics Book Archive is, rather obviously, a huge archive of old comic books.
If nothing else, this site proves - to me at least - that nostalgia ain't what it used to be...
Not quite as much fun as this fantastic archive of old advertising, it's still worth a glance.
The comics are archived in a special format, for which you'll need to use CDisplay or a similar application to read. Alternatively, Mac OS X and Linux users can use Comical.
Found via Digg.