Even after Sutcliffe was arrested, signed a confession and was locked away for good, some people believe that the Ripper hadn't actually been caught. This belief is most forcefully and eloquently put forward by Noel O'Gara. For full details, I recommend reading O'Gara's own website, although I can offer a summary below, and some of the objections to his theory that have been raised.
O'Gara maintains that Sutcliffe was a deluded 'copycat' killer who was, in fact, responsible for only four of the canonical Yorkshire Ripper series of murders. He draws on an extensive catalogue of contemporaneous press reports to back up his belief that his former employee - one William 'Billy' Tracey - was responsible for the majority of the murders ascribed to Sutcliffe.
It is true that the police issued many contradictory statements about the Yorkshire Ripper over the long years of their investigation. Among the detectives, there were arguments about which sources were credible, which attacks had been carried out by the Ripper and what methods should be used to catch him. Most famously, of course, was the derailment of the investigation by the hoax tape. On the basis of this alone, detectives were told to eliminate any suspect who didn't have a north-eastern accent.
O'Gara holds that the identity of the 'real' Yorkshire Ripper lies in the repeated mentions of a bearded Irish suspect in police pleas for information - many of which he reproduces on his website. The police often made mention of a 'copycat killer' during their investigation, in the belief that some murders at the time had been committed by another hand.
Notable among these, for example, was the murder of Joan Harrison in Manchester. At the time, this was widely attributed to the Yorkshire Ripper - both by the press and by some of the investigating detectives. In fact, Harrison's killer was finally unmasked by DNA analysis in 2011. The suspect died three years previously but shortly before his death had a DNA swab taken in connection with an arrest for drink-driving. A note written by the man the day before he died contained what appeared to be a confession to the murder.
In the pre-DNA era, crimes in a series could only be linked by witness statements - which are notoriously variable in reliability - and the M.O. of the killer. The Yorkshire Ripper was known to kill with a hammer and mutilate the body after death. Thus any death which appeared to match those conditions could be considered a Ripper crime.
Moving from the specific to the general, the police interviewed tens of thousands of men in their investigations and made appeals to help identify thousands more that went unanswered. Shortly before her death, for example, Wilma McCann was seen getting into the car of a 'West Indian' man - and despite appeals and a widely-distributed description of both the man and his car he was never identified.
Thus, to those who doubt O'Gara's theory, the appearance of various suspects in the media - such as the bearded Irishman - are effectively inevitable red-herrings thrown up by a vast police operation in which countless thousands of men could be potential suspects.
Another pillar of O'Gara's theory is the undoubted truth that West Yorkshire police were quite prepared to see innocent people convicted. Notoriously, Stefan Kiszko was imprisoned for 22 years for the murder of Lesley Molseed - a crime later conclusively proven to have been committed by Ronald Castree. Intriguingly, the lead detective on this case was Dick Holland - who was a senior figure on the Ripper investigation. The similar conviction of Judith Ward for the 1974 IRA M62 coach bombing was predicated largely on the work of George Oldfield - the senior (and certainly most publicly visible) detective on the Ripper investigation.
O'Gara presents the theory that the police - driven to desperation - found in Sutcliffe an ideal 'patsy' who was prepared to confess to being the Yorkshire Ripper in return for being found 'insane' and thus avoiding prison. Some have argued that this is effectively a conspiracy.
See also: conspiracy theories