In Perfection, they say there's nothing new under the sun. But under the ground...
The horror/comedy film almost never works, because it requires a nearly impossible tone to nail. It's hard to be too scared if you're laughing, and it's hard to laugh if you're scared. When writers and directors do pull it off -- Gremlins 1 and 2, the Evil Dead series, the recent Slither -- it's an incredibly enjoyable genre. And to me, the shining example may just be the 1990 cult classic Tremors.
The film is about underground creatures that track their prey by sensing vibrations. It's a pretty genius idea for a horror flick, one of those perfect why-didn't-anyone-think-of-this-before concepts. Having the creatures come from below is something of a masterstroke for a low-budget film, because for large chunks of screen time the monster can be implied rather than shown.
The smaller budget of Tremors pushes the filmmakers to be as creative as possible with their monsters. In addition to the awesome cinematography, which includes Sam Raimi-style camera tricks and monster POV shots, just about every creature feature trick in the book is employed, including hand puppets! There are only a couple of moments that don't quite look believable, but the shagginess is a big part of the movie's massive charm.
And the monsters, when we do see them, are really pretty sweet. There's a great documentary on the DVD where, among other things, you learn that the original creature design was scrapped because everyone thought it looked exactly like a penis. So even if the effects might not be up to today's CGI-heavy standards, you can at least be thankful you're not watching Attack of the Dicks.