I know he stopped one in these:
1.) Mr. Monk and the Class Reunion
2.) Mr. Monk and the Bully
3.) Mr. Monk Is On the Run Part 2
4.) Mr. Monk Is Someone Else
5.) Mr. Monk Is At Your Service (saved Natalie)
6.) Mr. Monk Goes to a Wedding (the bride was going to kill the groom later on.)
7.) Mr. Monk Takes a Punch
8.) Mr. Monk and the Paperboy
9.) Mr. Monk and the Other Detective
10.) Mr. Monk and the Actor (technically he stopped the actor from killing the suspect)
11.) Mr. Monk and the Naked Man (this one is debatable if it counts as a murder attempt, though as Stottlemeyer said that if the guy died, the perp would be charged with two deaths, I think I'm safe in counting it as a murder attempt, where Monk uncovered a health danger to a CEO that the killer of another person was trying to hush up, which no doubt saved the guy's life)
12.) Mr. Monk and the Daredevil
13.) Mr. Monk Is Up All Night
14.) Mr. Monk and the Three Pies (the bad guy set Ambrose's house on fire and his brother could well have died had Monk not come.)
15.) Mr. Monk Goes Home Again (it's possible that Ambrose (or a Halloween trick or treater from the following year) may have eaten the poisoned candy bar had Monk not solved the case)
16.) Mr. Monk and the Voodoo Curse (Saved Natalie)
Am I missing any episodes where he stopped a murder?