1. In Pearl, Mississippi in 1997, 16-year-old Luke Woodham stabbed and bludgeoned to death his mother at home, then killed two students and injured seven at his high school. As he was leaving the school, he was stopped by Assistant Principal Joel Myrick, who had gone out to get a handgun from his car. I have seen sources that state that Woodham was on the way to Pearl Junior High School to continue shooting, though I couldn't find any contemporaneous news articles that so state.
2. In Edinboro, Pennsylvania in 1998, 14-year-old Andrew Wurst shot and killed a teacher at a school dance, and shot and injured several other students. He had just left the dance hall, carrying his gun -- possibly to attack more people, though the stories that I've seen are unclear -- when he was confronted by the dance hall owner James Strand, who lived next door and kept a shotgun at home. It's not clear whether Wurst was planning to kill others, would have gotten into a gun battle with the police, or would have otherwise killed more people had Strand not stopped him.
3. In Winnemucca, Nevada in 2008, Ernesto Villagomez killed two people and wounded two others in a bar filled with three hundred people. He was then shot and killed by a patron who was carrying a gun (and had a concealed carry license). It's not clear whether Villagomez would have killed more people; the killings were apparently the result of a family feud, and I could see no information on whether Villagomez had more names on his list, nor could one tell whether he would have killed more people in trying to evade capture.
4. In Colorado Springs in 2007, Matthew Murray killed four people at a church. He was then shot several times by Jeanne Assam, a church member, volunteer security guard, and former police officer (she had been dismissed by a police department 10 years before, and to my knowledge hadn't worked as a police officer since). Murray, knocked down and badly wounded, killed himself; it is again not clear whether he would have killed more people had he not been wounded, but my guess is that he would have.
5. The Pearl, Mississippi school shooting was stopped by the vice principal Joel Myrick with a Colt .45, The Appalachian School shooting was stopped by two students with handguns. Both of the above incidents were stopped by the armed citizens threatening the shooter without firing.
Pearl High School
6. Appalacian Law School
Plans to slay everyone in the Muskegon, Michigan, store and steal enough cash and jewelry to feed their "gnawing hunger for crack cocaine" fell apart for a band of would-be killers after one of their victims fought back.
7 Muskegon Shooting
The mass church shooting in Colorado Springs was stopped by the shooter being shot by a church member with a CCW permit.
8. New Life Church
The Santa Clara gunshop shooting in 1999 was stopped by an armed citizen after the shooter declared that he was going to kill everyone. Police found a list of intended victims in his car. Only the perpetrator, Richard Gable Stevens was shot.
10. Santa Clara Gunshop
The December, 1991, Aniston, Alabama defense where a CCW holder stopped armed robbers who were herding employees, customers, and his wife into a cooler. He shot both robbers, killing one.