It seems like as soon as a “big” movie hits the video store, an unknown knockoff (Alien Vs. Hunter is one of the most blatant) appears. I know some places make their money this way. Hell, everyone seems to have jumped on the Saw bandwagon since it started. Are any of these even good?
Battle Beyond the Stars is a pretty good little movie, even if it’s a knockoff of Star Wars and The Magnificent Seven*.
There are also quite a few successful US films that are knockoffs of foreign films. There are six or seven knockoffs of Yojimbo alone. There’s also Three Men and a Lady, and The Birdcage.
*Which was a knockoff of The Seven Samauri.
Maybe not exactly what you’re looking for, but in the years following the release of Die Hard, there was a slew of movies involving terrorists taking a bunch of hostages and making demands, with some maverick hero-type thwarting their efforts. Hence, Speed was sometimes called “Die Hard on a Bus” and Under Siege was “Die Hard on a Boat” and Executive Decision was “Die Hard on a Plane.” They were all pretty good action movies in their own right. But I don’t know if they should really be called knock-offs. So I guess I got nothin’. 
Romancing the Stone is a good film; certainly better than the other “Raiders” knockoff, High Road to China.
Major League came out shortly after Bull Durham and it’s a lot of fun.
And High Road To China isn’t bad at all.
One of my all-time favorite movies is a cheapo Animal House/Porky’s clone called Up The Creek. It starred two Deltas, PeeWee, a bunch of Bunnies and James B. Sikking.
Hmm. I’m wondering if “remake”–credited or not–is the same thing as “knockoff.”
I’d see them as two different things.
That, and other films of the like, are put out by The Asylum, and intentionally target to release when the big-name releases theatrically. They’re currently being sued by Fox over their upcoming The Day The Earth Stopped. The press has dubbed Asylum’s similarly-titled cash-in works, such as Snakes on a Train and Transmorphers, “mockbusters.”
Gremlins inspired a slew of bad knockoffs, but Critters was a pretty fun movie that stood well on its own.
Well, Battle Beyond the Stars was thus technically a knockoff of Star Wars and a remake of The Magnificent Seven. It was made to cash in on SW, but the plot (by John Sayles) was right out of TM7.
The Last Starfighter was also a Star Wars knockoff, with hints of The Music Man.
There were many 50s knockoffs of Them! featuring giant insects, but Tarantula is actually a pretty good movie.
The oxen are slow, but the earth is patient.
I confess to adoring that movie. That and “Tales of the Gold Monkey” which was similar.
And then there’s my favorite Raiders knockoff, The Perils of Gwendoline
…
What?