Matt, add me as another one who has seen
They Shoot Horses Don't They?. My older sister took me to see lots of movies at the drive-in before I was able to drive and this was one of them. Such an intense, grueling movie that is so well-acted. One that is not easy to forget.
More here that y'all have mentioned:
Agree that
Scott Pilgrim vs. the World is unfairly underrated. Such a fun movie.
The Apostle was also great. Duvall can do no wrong in my book. Love
Bring Me the Head of Alfredo Garcia,
The Life and Times of Judge Roy Bean.
Fear and Loathing in Las Vegas may be a bit underrated out there in the world, but it seems a lot of people here love it, as do I.
Land of the Dead---I can't understand the dislike for this awesome zombie flick...it's got everything you want from a zombie movie, plus the greatness of Dennis Hopper playing a Donald Trump-like millionaire.
For myself, I have to include two that are on my Top Ten list in my profile:
The Ninth Configuration (1980)...an almost forgotten movie from 1980 that is about a castle in the Pacific Northwest that is an asylum for soldiers during the Vietnam war who may or may not be faking their mental illness. It's up to a new psychiatrist to determine who is or isn't really ill. He indulges their quirks, and meets one patient in particular who challenges his authority and intrigues him to no end. But, the new shrink has quirks himself and acts almost as crazy as the patients. It's a drama, no doubt, but is riddled with hilarious dialogue and circumstances that are caused by the patients. Just for one example and not really a spoiler: one group of patients are attempting to put on a Shakespeare play that is cast solely with dogs.

This was written and directed by William Peter Blatty, who also has a cameo. I think this is the best thing he's ever done (yes, even over
The Exorcist). I highly recommend this movie.
Another overlooked movie:
Inside Moves (1980). The plot sounds simple enough: a failed suicide finds a bar near his rehabilitation center that is populated with men with various injuries, a lot of them permanent in nature. Although shy at first, he soon finds a home in the bar with this group of men who are all upbeat about their infirmities and joke all the time. He meets a man who becomes his best friend, the bartender, and their relationship is complicated by the bartender's self-pity and selfishness. He also meets a waitress he falls in love with. She's played by Diana Scarwid and she was nominated for Best Supporting Actress (but lost). The movie starts off as a downer but travels uphill all the way. One of the best films I've ever seen, mainly due to the acting and writing and directing by Richard Donner.