Not having anything particularly original to say, I have chosen to take my inspiration from Matt of the Holy Whapping's post on Louis XVII of France and the various pretenders who attempted to assume his identity.
The death of France's boy-king brings to mind the mysterious vanishing of Edward V of England and his brother Richard Duke of York. Following the death of their father Edward IV in 1483, control of the kingdom passed to the Protector Richard Duke of Gloucester. They were accomodated in the Tower of London whilst preparations were made for the coronation of of the 12 year-old monarch when questions arose as to the legitimacy of the children. Richard of Gloucester then arranged for himself to be crowned Richard III of England and the 'Princes in the Tower'. A couple of years later, Richard III was toppled from his throne and replaced by Henry VII, a Tudor with a very shaky claim to the kingdom. Thanks to Shakespeare's Richard III and St. Thomas More's history, the most commonly accepted version of events puts the blame for the murder squarely on the 'deformed' shoulders of Richard III, but the evidence is ambiguous. Various other theories have been proposed, such as the princes dying of natural causes, partisans of Richard III killing the princes without his knowledge or (very plausibly given what we know of the Tudors) their being murdered by Henry VII who say them as a threat to his throne and found blaming Richard very convenient for the purposes of propaganda. There are any number of websites out there dealing with the Princes, but of particular interest are those written by Ricardians who try to rehabilitate the last king of the house of York. (Particularly fun, if you're ever in York, is the dramatic apology for Richard put forward by local actor Michael S. Bennett)
What is without a doubt, however, is that well into the reign of Henry VII (1485-1509) there remained a huge doubt regarding the fate of the princes (there were tales of them living in Europe) and two pretenders to the English crown attempted to urge their claim on the basis of being one of the princes. Both Lambert Simnel and Perkin Warbeck both claimed to be Richard, Duke of York, younger brother to Edward V. There are several other similarities in their escapades - both emerged from obscurity and common birth, both suceeded in raising support in Ireland and both lead abortive rebellions against Henry VII.
Simnel was schooled by an Oxford priest in courtly manners, and after a period of claiming to be Richard, Duke of York, changed his story and as Edward, Earl of Warwick (nephew to Edward IV, first cousin of the princes in the tower and potential claimant to the throne) began his campaign for the English throne in Dublin. There, in 1487, at the age of 10, he was crowned Edward VI of England. The Irish, having Yorkist sympathies and always keen to cause trouble for an English king, backed Simnel militarily and also managed to secure some support from Europe, including a force of Flemish soldiers. Henry VII, unbeknownst to his enemies, actually had the real Edward of Warwick in prison and paraded him through the steets of London to demonstate Simnel's imposture. The Irish and Flemish force were quickly defeated after landing in England. Seeing that Simnel was merely the tool of older conspirators, Henry VII gave him a position in the royal kitchens, and he rose to become Royal Falconer before his death at the age of about 57.
Perkin Warbeck came from Flanders, and it seems that whilst in service to a merchant visiting Ireland something about his bearing attracted attention. Again, various Irish nobles of Norman descent, took him in hand. He moved in various court circles and by 1492 (aged about 18) he was summoned to Flanders to meet the sister of Edward IV (and thus aunt to the Princes) who gave him her support. Severa European monarchs also siezed the opportunity to discomfort Henry VII. Again, Warbeck seemed to have multiple personalities, being variously described as a bastard son of Richard III or a cousin of the princes, before it was finally decided that he was Richard, Duke of York. In 1495, aged about 21, he campaigned militarily in England and Ireland with little success, and so fled to Scotland. There, James IV arranged a marriage for him and he had some small military sucess in the North of England. Finally, in 1497, with a small force, he beagn his last campaign by landing at Lands End at the south-western tip of England. Unsurprisingly, the disgruntled Cornish folk who disagreed with Henry VII's taxation policy joined in the rebellion and he made progress through Cornwall and Devon until he reached the walled city of Exeter which was strongly Tudor. Unable to progress any further, he fled and eventually gave himself up. (In reward for their loyalty, the City Council of Exeter were granted a Cap of Maintenance and Ceremonial Sword by the grateful Henry VII. A number of years later, Exeter would become the stumbling block for the Cornishmen who formed the Catholic Prayerbook Rebellion.) Warbeck was imprisoned in the tower and in 1499 was dragged through the streets of London and hanged for attempting escape.