Raiders of the lost ark even had some, when the really tall man gets cut up by the plane propellers.

Braveheart

the Wrath of God sequence had an exploding head, obscured by smoke and light.

Starship Troopers

Boogie Nights
Magnolia

The 'half the head' decapitation is amazing, do you also know that the scene was in fact trimmed to receive it's R-rating. I would love to see the unedited shot in all it's bloody Verhoevenesque glory.

Never included in the theatrical cut of the film, but the head explosion in Scanners had a shot of the headless body lifelessly slumping to the ground that was edited out in post production.
I was really surprised when I saw the film again on early evening (non-cable) Minneapolis television that the scene was present and not cut

Fangoria had a truly sickening 'deleted scene' still of the machete to the head from "Friday the 13th Part 2". Someday we horror fans may or might not see the legendary uncut version of "My Bloody Valentine" where one female victim's head in impaled on a water pipe gushing out water and blood from out of her mouth.

RoboCop toward the end when the guy gets dowsed by toxic waste and then Boddicker hits him with the car.

There's also that movie called "Street Trash" where one character blows up in mutli-coloured goo.

and how can anyone not forget "ReAnimator'"s eye-plunging, head crushing, thwacked on the wall splatter - classic!

Braindead has several of them (actually, one of the heads continues to live and attack the protagonist).
Mars attacks

LolaRennt.

Fist of the North Star, practically everyone that dies in the movie head explodes into a fountain of guts and blood

granted it's anime, but for exploding heads it does not get any better than this

On the other end of the spectrum.. we've got exploding asses in "Men Behind the Sun", "Dreamcatcher" and "Society"

Maniac (1980) has by far the best exploding-head sequence I've seen. The make-up man himself, Tom Savini, played the hapless victim of a devastating shotgun blast that left behind nothing but viscera and flapping skin.

Savini also did the SFX work (another particularily nasty shotgun blast to the head) in the slasher flick "The Prowler" now seen in it's full uncensored glory on BLUE UNDERGROUND's DVD edition.

Just got reminded of a shot from the japanese zombie/yakuza film "Versus." It has a (shotgun I think, or some huge gun) blast leaving a gaping hole from front to back of some poor fellow's head. The camera turns from front to back, through the hole to the shooter, and the most disturbing thing is the persistent tooth hanging at the bottom of the "head tunnel."

It seemed taken from some artwork Geof Darrow did for his collaboration with Frank Miller on "Hard Boiled."

Thin Red Line had an exploding ass

Yes, you're right! At the film's finale; that was a very good one too.

Hamburger Hill - some NVA soldier takes an M-60 blast in the head.