I read a science fiction book once where there was a suicide booth which put you in a large centrifuge and spun you until the G-Force was so powerful that the blood in your body couldn't overcome it and you had a stroke. I can't remember which book that was.
I think there's fairly good science behind it. Lots of people have passed out from excess G-forces and they don't describe it as pain, as such. And once you pass out I would presume that the force providing for death aren't noticeable. But then again, anyone who has died from it can't tell us. (Because they're dead.)
See, it looks like a super-fun way to commit suicide to me. I'd love for my last word to be "Wheee!!!!!".
Some design student made/designed a rollercoaster of death, with that exact intention. I think it was where I read about the g-force death.
A YouTube mirror for people who have trouble with Vimeo for whatever reason (e.g. me): Bonus: in the bit at 2:43, watch the middle towards the bottom.