Well, okay, here goes:
Jack the Ripper: madman's reign of terror in Victorian England's Whitechapel district in 1888 results in five canonical murders and yields a wealth of possible suspects which have fascinated for the last 124 years.
Tylenol Poisonings: in late 1982 in the Chicago area, seven die after taking poisoned over-the-counter Extra Strength Tylenol capsules.
Black Dahlia: would-be actress disappears and is discovered early one morning in 1947, in a vacant lot, hideously disfigured, severed at the waist, drained of blood, and washed and cleaned.
Zodiac: in California in the late '60s and early '70s seven die at the hands of a killer whose cryptic messages to newspapers also state his claims to having killed 37 people.
Taman Shud: the death of an unidentified man on Somerton Beach near Adelaide (Australia) in 1948 is unsolved; an undetected poison, Cold War tensions, and a possible love interest make this a great mystery 64 years on.