Other Suspects

Montague John Druitt
Born in 1857, one of 7 children, Druitt was a schoolteacher and lawyer. A keen sportsman, he was the Treasurer of Blackheath Cricket Club.
With very little evidence to support the theory, Sir Melville Macnaghten effectively accused Druitt of being Jack the Ripper, (a) because he assumed him to be a doctor (b) because he committed suicide shortly after the 'last' killing in 1888. Conspiracy theorists have suggested Druitt was the 'patsy' - set up to take the blame, and possibly murdered himself. Daniel Farson suggests that in Australia there is documentation linking Druitt to the Whitechapel murders.

Kosminski
Another Macnaghten candidate was a Polish Jew named Kosminski. Despite being a comparitively rare name, there are at least 3 'Kosminsky' names to add to the confusion. In 1877 an application for naturalisation was made by a Martin Kosminski, supported by Joseph Hyam Levy - one of the witnesses to an attack on Catherine Eddowes on her fateful night. If the attacker was this Kosminski, Levy must surely have recognised him?
Aaron Kosminsky is the only person of that name in the Colney Hatch records (admitted in Jan 1891), transferring to the Leavesden Asylum in 1894, dying there in 1919.

A Nathan Kaminsky, (Kosminski?) who lived in the same area as Leather Apron/John Pizer, was discharged from the Whitechapel Infirmary in May 1888. No record of his death exists (he may have moved outside England, changed his name etc), but some people have suggested he is the same person as David Cohen, arrested by the police when found wandering the streets of Whitechapel in December 1888. He was found to be dangerous to himself and others and sent to Colney Hatch Asylum in December 1888, where he died in October 1889.

Anderson and Swanson appeared to support the Kosminski theory - In his autobiography, Anderson says "the only person who had ever a good view of the murderer unhesitatingly identifieds the suspect the instant he was confronted with him; but refused to give evidence against him". Swanson anotated "because the subject was also a Jew and also because his evidence would convict the suspect, and witness would be the means of murderer being hanged" and also "And after this identification which subject knew, no other murder of this kind took place in London" and at the end he wrote "he was sent to Stepney Workhouse and then to Colney Hatch and died shortly afterwards - Kosminsky was the suspect - DSS".

John Pizer 'Leather Apron'
A Jewish shoemaker and slaughterman, known to have used knives and a leather apron and possibly extracting money with menaces from prostitutes, Pizer was an early suspect. Press articles virtually accused him of being the Ripper. But he demonstrated strong alibis, and ultimately sued and received compensation for the stories.

Michael Ostrog
Another Macnaghten 'suspect' - "a mad Russian doctor and a convict and unquestionably a homicidal maniac", who carried surgical knives and was habitually cruel to women - with very little supporting information or facts. Supposedly detained in a lunatic asylum and still alive, when Macnaghten wrote in 1894, few traces other than MacNaghten's notes has been found of this person. Though there has been a story he died in Russia under the name Konovalov.

Royals The 'Conspiracy Theory'
There are a number of conspiracy theories involving Prince 'Eddy' (Duke of Clarence)

Walter Sickert (Painter)

Joseph Barnett
Mary Kelly lived with Joseph Barnett at Millers Court; and even at the time it was suspected hers was a 'copycat' killing, and Barnett was intensively questioned. There are a number of key issues in the Kelly killing: it was indoors, in a locked room, she may have been seen between 08:30 and 10:00am - when she was supposed to have been killed at 04:00am (and if so, Barnett's alibi may not hold true), Barnett's pipe was found at the scene (though he was supposed to have stopped living there) - and Robert Anderson claimed this was one of only 2 "real clues" (the other being the Goulston Street graffiti).
If Barnett killed Mary Kelly, did he also kill (at least some of) the other victims?

George Chapman
"You've got Jack the Ripper at last!" - Abberline is supposed to have expressed to Inspector Godley after the conviction of Chapman for his 3 'wife' murders in 1903.
Real name Severin Klosowski (could this have been another police 'Kosminsky' with the primitive standards of record-keeping?), born in Poland in 1865, he was apprenticed to a surgeon (Moshko Rappaport) in Zvolen until 1885. From there he went to Warsaw, and by 1887 was living in London as a hairdresser. At the time of the murders he had lodgings in, Whitechapel. He emigrated, with his wife Lucy, to New York in 1891. He attacked Lucy with a knife, and she fled back to England in early 1892; with Chapman re-appearing himself about the beginning of June. Chapman was almost certainly in the USA at the time of the Carrie Brown killing.
The strongest argument against Chapman as the Ripper may be that he poisoned his 'wives' rather than using a knife.