The Fly is a 1958 horror movie starring Vincent Price. David Cronenberg remade this in the 1980's with Jeff Goldblum.
Youtube has this online in pieces. Part one has the embedding feature disabled, but it can be viewed at this youtube link. The other parts should autoplay from part 2 below:[these have all been removed. I don't understand why copyright holders think films divided up into short segments like this can possibly hurt them.]
Here's a trailer:
For now [as of 9/10/2012) it's online here:
Moria describes it as "one of the most memorable of all fifties sf/horror crossovers. ... a classic and a rather fine monster movie." 1000 Misspent Hours calls it "one of the most famous sci-fi/horror movies of the 1950’s" and says, "The importance of this movie from a B-horror fan’s perspective ought to be fairly obvious. But The Fly is also significant for the development of cinematic science fiction as a whole." The New York Times says it "happens to be one of the better, more restrained entries of the "shock" school."