Them!

Despite the kind of goofy title, and a somewhat ridiculous premise, this movie is actually really good, and I recommend it for anyone looking for an example of a 1950s horror/sci-fi crossover.  The story, acting, and even the special effects still hold up pretty well.  The ants themselves are fairly primitive compared to today's effects standards, but they are handled in a way that isn't too cheesy, and there are actually a few good scares.  The plot is structured more like a detective story than a simple man vs. monsters battle, which is pretty interesting.  It does suffer a little bit from the pacing issues that seem to be present in almost all the movies we've watched, where it's pretty tense for the first twenty minutes, meanders for a while, and then rushes to wrap everything up in the last two minutes.  Also, like many 50s sci-fi films, the ants were created by nuclear radiation, and the underlying theme is sort of an allegory/warning against the fears of the atomic bomb and the Cold War.