The Giant Spider Invasion
Bill Rebane's backyard wonder is a hilariously hokey throwback to the giant-monster flicks of the 50s, while taking its only pseudo-scientific conceit from the mid-70s trend of popular black hole theories, combining these two elements to create pure bad-movie heaven. A tiny, decrepit town in rural Wisconsin is rocked by a mysterious explosion, after which a drunken farmer (Robert Easton) discovers dozens of crystal geodes strewn across his property.
Cracking each one open reveals a diamond-like encrustation (which to the farmer's dismay proves nearly worthless), as well as a hairy but decidedly un-cosmic tarantula.
Being permanently soused, he fails to notice the latter (his equally boozy wife accidentally purees one in her daiquiri!) until the rocks begin opening on their own, and the entire farm is overrun with eight-legged invaders.
Despite the title, only one of the spiders would qualify for giant status, but it's definitely a keeper: the Big Daddy arachnid is quite clearly a Volkswagen Beetle covered with black fake fur, with legs operated by the car's passengers and taillights doubling for glowing red eyes.
Unfortunately, it is merely glimpsed through much of the film, making its triumphant appearance in the final few minutes.