This question has been asked way too many times. Credits go to /u/zirtbow for the summary.

•Black Dahlia •Original Night Stalker •Lead Masks Case •Taman Shud •Dyatlov Pass •Severed Feet of British Columbia •Hinterkaifeck Murders •Circleville Writer •Zodiac Killer •Disappearance of Amy Lynn Bradley •Who put Bella in the Wych Elm •Texarkana Moonlight Murders •Disappearance of Tara Calico •Death of Michael Hastings •Villisca Axe Murders •Oklahoma girl scout murders •Disappearance of Madeleine McCann •The Gilgo Beach Killer •The Alphabet Murders •Isdal Woman •Tylenol Poisonings •D.B. Cooper •Disappearance of Ray Gricar •The Mad Gasser •Disappearance of the Beaumont children •The Family Murders (from above) •The Max Headroom Intrusion •Smiley Face Murder Theory •Boy in the Box •The Colonial Parkway Killer •Brabant killers •Keddie Murders •West Memphis Three •The Monster with 21 Faces •Assassination of Olof Palme •Ken Rex McElroy •Lake Bodom murders •Cleveland Torso Murderer •Noida Double Murder Case •West Mesa Murders •Disappearance of Jim Gray •Danny Casolaro Suicide •The Springfield Three •Servant Girl Annihilator •Axeman of New Orleans •The Monster of Florence •Flannan Isles Mystery •The Atlanta Ripper •JonBenét Ramsey •The Mary Celeste •Frank Olson •Seewen murder case •Jimmy Hoffa •Death of Gareth Williams •Disappearance of Jim Thompson •Disappearance of Richard Colvin Cox •Isabella Stewart Gardner Museum Art Theft •Hello Kitty murder •Girl A •Freeway Phantom •Oakland Child Killer •Santa Rosa Hitchhiker Murders •Bowling Alley Massacre •The Black Doodler •Eastbound Strangler •Walker Family Murders •Disappeared •Connecticut River Valley Killer •Wanda Beach Murders