The History of the Metropolitan Police: Historical overview of the Metropolitan Police Force from 1829 to the present, including major cases.
Stories from Scotland Yard: Numerous anecdotes about cops and criminals, courtesy of the authors of the Encyclopedia of Scotland Yard.
The Victorian Policeman: From the Bow Street Runners to the Peelers.
Police History: History of the Irish National Police.
Police Notebooks: Charles Booth's notebooks on his walks with the London police, frequently in slum areas.
"On Duty with Inspector Field": Dickens' narrative of a night with Inspector Charles Field.
The History of Crime and Punishment in Britain 1790-1870: University of Glasgow special exhibit, featuring digitized broadsides.
Curiosities of Street Literature: Digitized edition of this 1871 anthology, which includes gallows speeches, "shocking" accounts of various murders, etc.
Murder in the UK: If they killed someone, somehow, they're probably here somewhere.
Notorious Criminals: Criminal Scots.
Capital Punishment UK: Extensive coverage of the death penalty's history in the UK, with accounts of significant criminal cases and major prisons.
Tyburn Tree: Execution in the sixteenth and seventeenth centuries; includes images, various speeches from the gallows, and related e-texts.
Victorian Crime and Detection Themes: Unpleasant happenings in the Shropshire area.
North East History: Crime: Historical accounts of major crimes in and around Durham.
Dr. William Palmer: Notorious Victorian poisoner, hanged in 1856. Both this site and this one cast doubt on Palmer's guilt. Dr. Palmer was the subject of a 1998 miniseries.
Mary Ann Cotton: Another (in)famous poisoner, responsible for at least twenty-one deaths.
Dr. Thomas Neill Cream: Yet more poison.
Maria and Frederick Manning: A street ballad. See also this account of their execution from the Anglo-Celt (scroll down).
Amelia Dyer: A baby farmer.
Constance Kent: Study of the street ballads devoted to the teenage murderer. See also Charles Spurgeon's sermon on Kent; here's a photograph.
Madeline Smith: Website devoted to the young woman accused of killing her lover.
Casebook: Jack the Ripper: Everybody's favorite Victorian serial killer.