From the 5/17 IMDb poll:
Gotta go with a recent one:
The Green Goblin, in an idiotic attempt to kill Spider-Man, remote-controls the Goblin Glider directly into his own crotch.
I’ve gotta go with a favorite that is also somewhat “D’oh!” embarrassing. Christopher Lee, perennial villian, in The Man Withe the Golden Gun, a James Bond movie. He gets his comeuppance in a way that makes this Roger Moore Bond film my all time favorite.
Christopher Lee trivia:
Has been in hundreds of films.
Tallest leading man in a Hollywood movie.
Almost 80 and wields a wicked lightsaber in Star Wars and plays the baddie in Lord of the Rings, The Two Towers, albiet with less action than the first movie.
Speaking of Bond movies, you can’t beat the air compression thingy that got inserted into the lead villian and blew him up in You Only Live Twice.
Wanna tell us how he actually dies?
From Return of the Jedi:
I have never liked how Boba Fett was killed off in that movie. Getting his jetpack accidentally ignited and ricocheting off the sail barge into the Sarlacc pit looked like such an undignified death for a character that’s supposedly a bad-ass.
Wicked Witch of the West…she must have stuck if she could never take a bath. At least her sister had to have an entire house dropped on her head to put an end to her madness.
(anyone ever wonder if a munchkin or two weren’t landed on as well?)
and of course I meant she ‘stunk’ not stuck…unless you threw hew undies against a wall. 
Atreyu- who says he died?
stankow:
That’s exactly how the comic-book version died. Long-time Spider-Man fans (or me, at least) could see it coming as soon as we saw Osbourne surreptitiously remote-controlling the glider.
Buzzsaw, from Running Man. Ah-nold: “He had to split.”
If we’re including any movie bad guys under the term “villian,” and not just the main antagonist, I have to go with a scene in Out of Sight. White Boy Bob, one of Snoopy’s (Don Cheadle’s) gang, is coming down the stairs pointing a gun at George Clooney, when he trips, falls down, and accidentally puts a bullet through his own brain.
Bryan Ekers, I remembered that it happened that way, but I didn’t recall the exact, erm… point of entry. Perhaps it was just more noticeable on a large screen.
Forgive my ignorance, but how are you supposed the read the spoilers?
Okay, I figured it out. Never mind
Well, I haven’t figured it out. What do I have to do?
Drag your mouse over 'em, a90210.
Just hilight the black area with your mouse, or doubleclick. Then the text is white and the background is blue.
Oh! Okay, thanks.
I nominate Bullet Tooth Tony from Snatch:
When the dog grabs the diamond from Avi, Avi grabs Tony’s gun and starts shooting wildly around the room, killing Tony. The dog escapes through a window.
If you use Opera, change the style sheet.
This isn’t a death scene, but in The Simpsons a waiter was suing a playboy for allegedly damaging him. Bart finally decided to testify that it was an freak accident. The waiter angrily snarled that he wasn’t that clumsy, then proceeded to have another freak accident culminating in him going out through the window.