It's interesting to note that, while we have several movies
from the 50's, two from the 30's and one from the 60's, there
nothing here from the 40's. Apparently it was a bad decade for
Science Fiction/Horror flicks.
Note, also, that most of the movie cited are rather short,
generally running only about 80 minutes. This is because, true to
the title, most of these films were intended to run as part of a
Double Feature, and the theater owners wanted to get two of them
in, along with a cartoon & a newsreel, in under 3 hours.
The information provided here was gathered from "Microsoft's Cinemania" CDROM, Baseline's "Motion Picture Guide" and "Encyclopedia of Film", and a youth misspent watching a lot of late-night TV.
Many link access The Internet Movie Database at http://www.imdb.com/Movies
However many links go elsewhere, so be sure to poke around a bit.