Flash Gordon
The director may not have known it was stupid, and whoever was playing Flash certainly didn’t know it was stupid.
But everyone else involved knew exactly what sort of movie they were in, and were obviously camping it up for the fun of it.
From the StompTokyo review:
Noo – it was F-ing Brilliant!
I love that film.
The Librarian - the Quest for the Spear. I loved that movie. Basically it’s Indiana Jones, but the hero is a librarian, who is an extremely nerdy perpetual student, and who saves the day by feats of academic derring-do.
D.E.B.S. was pretty funny, too.
A rip-off of Animal House and Porky’s, starring actors from both, UTC manages to be both serious at being a comedy and poke fun at the genre at the same time. One of my all-time guilty pleasures.
See also The Faculty and Desperado 
I LOVE that movie, although it helps that I’m tearing through a Library Science program right now, hoping to become “The Librarian” myself.
I agree with Sublight on Flash Gordon. In which case the (stylistically very similar) Barbarella, Queen of the Galaxy warrants a mention.
One makes you laugh with it, the other makes you laugh at it. (With a lot of experience watching MST3K-grade fliks, one learns to tell the difference between the chuckle you get at a line like “You see, you see? Your stupid minds! Stupid! Stupid!” and a line like "I will not be distracted by your all-girl lesbian sex orgy…we must kung-fu fight!")
Anyway…
SS Doomtrooper (A Sci-Fi channel original. Basically Wolfenstein the Movie.)
I’m not sure if Dog Soldiers and the first two (or three) Return of the Living Dead movies count as “B-Flicks having a damn fun time of it,” or just outright good movies.
Same with Resident Evil: Apocalypse and the Reanimator movies. (Well, the third one would count as “kind of mediocre”…if not for the music video included on the DVD. “Move Your Dead Bones!” 
Heh. OK, it gets into heaven for that one. Check YouTube for details.)
And a hearty second to The Librarian.
At the risk of garnering some wrath, I’m gonna say Starship Troopers. Yes, I know the original story was this deep message on blah blah blah and the movie was an evil bastardization of the blah blah blah and Heinlein is rolling over in his blah blah blah. Didn’t read the book, not planning on reading the book, don’t care. The movie was hilariously stupid.
How about Red Dawn? I don’t think the makers intended it that way, but it sure is good stupid fun.
My favorite part is when invading Russian paratroopers storm a Colorado high school. (Because of course, that would be the first target of any foreign invasion.) Now that’s good stupid!
It’s been a while, but aren’t they invading Cuban paratroopers?
Eight Legged Freaks. Gotta love a Giant Killer Something movie. It’s quite clear that everyone knew it was a ridiculous movie, and they had fun with it. I mean, come on, the dying gasp of the spider that crawls out of the burning wreck of the semi? That’s pure Looney Toons, man!
Tangent already named mine – Big Trouble in Little China, in which Kurt Russel plays a hero who’s a jerk, but doesn’t know it.
Attack of the Killer Tomatoes is another. As someone asked when it first came out – “If someone sets out to make a Bad Movie and succeeeds, is it right to blame them?” The first sequel, by the way, was brilliant (even if you don’t count the participation of a not-yet-famous George Clooney).
It was. I’ve been thinking about starting a thread about this movie, as I think it’s really underrated. No, it’s not a masterful piece of classic cinema, but it’s a lot better than it’s given credit for.
Army of Darkness was the one I came in to mention. It’s hilarious and ridiculous, as close to a straightfaced parody of the action movie genre as you can get without actually becoming a parody.
“Name’s Ash. Housewares.”
I’d nominate They Live if I weren’t fairly sure none of the people involved knew how dumb it was.
All the Troma films probably qualify.
Heh, heh.
I just got through acting in one. It premiers in November (if you’re in Houston, you’re invited ).
The director set out to poke fun at the cheezy martial arts movies of the 70s as well as cheezy elements (in real life) of the martial arts industry, as well as cheezy infomercials. He succeeded nicely. We all had a great time hamming it up while we shot the movie.
“Doc Savage- Man of Bronze” was on the ragged edge between a real pulp adventure flick and self-parody. Doc: "Mona, you’re a brick! "
strange Invaders probably qualifies. One of my friends pointed out the wonderful absurdity of the alien spacecraft operated by what seems to be steam pipes, leaking dripping water from huge wheeled shut-off valves.
How 'bout * Plan Nine from Outer Space?*
Or the one with…I think it was Rosie Greer…, who had his head attached to some white guy? Both alive with one body.
Hilairous!
By all accounts, that was made in perfect seriousness. So was Manos Hands of Fate.