I've seen a number of accidents, but of the ones I wasn't involved in, two were absolutely astounding...
the first was when I was jogging with my team (cross country) and we were running forest trails. The top of one trail ended on a road, which we would follow for about a quarter of a mile to reach the trail we were going to run down. This road, called Skyline, ran the top ridge of range, and was smooth, curvy, and a favorite for motorcyclists. Most of the bikers up there were smart...the group we saw weren't. One guy rode past us heading south and just as he was approaching a bend, two idiots who had decided to race north came flying around. One racer clipped the innocent rider and flew into his racing buddy. All three riders went down in spectacular fashion, all three bikes being destroyed. Luckily, the innocent rode his leathers off into a berm and actually got up and ran over to the racers screaming...the racers, however, were pretty much done. Both were taken away in ambulances, and neither moved the entire time we were there.
The second accident was on a late-night drive up from LA. Interstate 5 is a straight line of pure boredom through some of the most featureless landscape. My friend was driving, I was in shotgun, and we were pacing behind a pick-up truck for a while. The truck started to drift a bit and swerve back onto the road. At first we thought he was drunk or was searching for a tape or some stupid shit like that. About the third time it happened, we realized the guy must have been dozing off...all of a sudden, he swerves again, this time all the way off the road. He must have jolted awake again, because the truck lurched left, went shooting across the inside lane, and hit the center divider, sending the thing rolling. My friend swerved into the slow lane, and just for a moment, we just looked out his window and watched this rolling truck keeping pace with us. It must have been just a second, but it was truly surreal...felt like it went on and on. Driver was banged up something fierce, again, taken away in an ambulance, we gave our statements and finished up our trip.