Some more cult classics [SPOILERS!]
Well, I think that one of the big cult classics of its time was the (original) Texas Chainsaw Massacre. Its low-budget quality & look have, as some reviewers have said, given it a documentary-like, almost realistic look to the film (which I can understand with the look of the images, but not when it comes to the camera angles used in some cases). It also subtly works in real-world issues that were relevant at the time (the oil embargo of the 70's for example--the kids couldn't just get in their van & drive away because they neglected to get gas earlier, and the only gas station around was out of gas thanks to the embargo [albeit other potential motives for this come to light later on in the movie]). The movie sort of has a happy ending with 1 of the kids escaping with her life, though she doesn't necessarily escape with her sanity; also, the bad guys weren't truly thwarted/stopped, either--one dieddue to sheer luck/circumstance, but the others were quite capable of continuing their evil deeds.
In a way (IMHO), this film was so successful as a "cult classic" (considering that a bunch of UT students made the film, rather than being a product of one of the major studios at the time), that in a way, it moved from being a cult classic to being iconic. It lead to a number of sequels (of questionable quality), as well as a remake.
The original Night of the Living Dead is another great example of a cult classic which, in turn, becomes iconic for movies of its genre. It too touches on current issues of the day (racism, for example). It also lacks a truly "happy" ending; even though there is a major effort underway to get the zombie problem under control, none of the characters we are initially introduced to & watch for most of the film survive the ordeal (the last person's death being especially tragic). This film also has become iconic, and has spawned sequels and remakes of its own, as well.
IMHO, a modern attempt at making a current horror "cult classic" film would be Rob Zombie's House of 1,000 Corpses. To tell you the truth, I didn't care for the film when I first saw it in the theater--I felt like the arduous process it took to even get this film to theaters (long production & editing) had adversely affected the film. Also, it seemed to be more like a 90-minute music video rather than a movie with the way it was edited (not that surprising, considering who directed it). Then, I watched it again on DVD, thinking there'd be at least extra footage along with it, or possibly added back in to the film (ala the EE LotR films). The DVD version was the same--no director's cut, no deleted scenes, etc.
It was after watching it again that I started to catch on to this one. It obviously emulates a lot of the classic late-night horror films of yore--the stuff of Elvira & what not (something which is mentioned in the film itself, w/ the lil' Dr. Wolfenstein's Creature Feature Show on TVs during the beginning of the movie). It's a weird mix of gore & camp, much like a fair number of horror films shown on such late-nite shows. It doesn't have a happy ending. The film is made so that the viewer doesn't sympathize for the victims, and cheers on the villains.
I think House of 1,000 Corpses could be a cult classic, though part of me feels like it's tried to be/behave like one from the get-go. For me, I think a cult classic film just "is" one, and doesn't consciously "act" like one, if you get my drift. It seems that cult classics are just made as films, and gain the status later after they're viewed by the masses, rather than be made intentionally as a "cult classic" during the process of making the film. Then again, time will tell for House of 1,000 Corpses, as well as for its sequel, The Devil's Rejects (which, IIRC, is supposed to be less campy and more gruesome than its predecessor).
IMHO, I'd have to say that a fair number of films by Monty Python (or its members) are cult classics, or at least started off that way. The "Monty Python" films (Holy Grail, Life of Brian, Meaning of Life) are rather big, though usually 1 ranks highest among all 3 for most folks (IIRC, Grail is the most popular in America, while Life of Brian is the most popular in Britain [if not Europe]). However, other films by various Pythons (as actors &/or directors) also wind up beign cult classics, IMHO: Gilliam's Brazil, Adventures of Baron Munchausen, Time Bandits, and 12 Monkeys; Jones' Eric the Viking; Palin's Jabberwocky, the Ripping Yarns series, and The Missionary; Cleese's A Fish Called Wanda; Chapman's Yellowbeard; Idle's The Rutles; etc.
Then again, that's just MHO on the matter.