Yeah, we're still rolling with the や行 over here, and today that means a great word to know for your Japanese traffic accidents, subway suicides, dolphin slaughterings, arsons... what else is morbidly entertaining?
野次馬Indulge me by letting me give you a quick explanation of where this one comes from. The 野次 bit can be used on it's own to mean to heckle or to jeer (used like this: 野次を指す） but they're actually 当て字, chosen to fit the meaning of this phrase a little bit better.
Spectators of something not intended for spectating (see explanation below).
Baseball game: × Motocross rally: ×
House fire: ○ Car accident: ○
The original phrase actually began as 親父馬 （おやじうま; old man horse) which was what you called a horse that was too old to be used for either riding or labor, but which, for some reason, you haven't yet shipped off to the 糊 factory.
An old useless horse, just standing around taking in the scenery then became a metaphor for people with no business being involved in something, loafing around, scoping it out anyway. It kind of connotes blowing off your own responsibilities (responsibilities like driving your car at a reasonable speed) to indulge in something undignified (gawking at a flipped k-car on the side of the road, for example).
I'm not sure if rubbernecker, or rubberneck is an Americanism, but now that I've been in Japan, I can't ever hear it without thinking of the original rubbernecker, 轆轤首。