Son of Sam and Zodiac weren't snipers.
However, there HAVE been serial random snipers (rifle from a distance). Thomas Dillon randomly shot 5 hunters and fishermen in Eastern Ohio in the early 90s...he was a gun-obsessed nut, fired so many rounds a year he damaged his hearing, would drive around randomly shooting hundreds of dogs and cats (but had a regular job and family).
I also ran across an intriguing reference in the Washington Post to a serial sniper around Penn Station in Manhattan in 1984 that was never caught; killed 1, wounded 7. Only found ONE other internet reference to it, one of the victims...apparently case was completely bungled by the cops, resulted in 40 being fired. Very odd I can't find more on it.
I think there have been more of these random sniper murders than people think, but since no one of these guys has accumulated a big kill total, they tend not to turn up on one of the thousands of "true crime" ghoulish sites cataloging serial killers...and the fact that they don't involve sick sexual mutilations or anything also makes them less interesting to those web sites.