I made a thread like this in the game section for games. I think movies deserve a thread like this too, as there are just as many underrated movies as games.
My first pick would be Grandmas Boy.
It's slowly getting popular, but it was so underrated and still kind of is. It's hilarious. Adam Sandler's screw had their own movie and it was just as good as any of his. Not a classic like his, but great to watch. I was stoned when I first watched it and I never laughed so hard in my life. I've watched it plenty of times sober, as well as many of my friends and they love it. Defiantly a cool movie. But didn't last long in theaters or gets heard of enough. =D