Is it legal to publicly share leaked data (for example, twitter.com user emails, names, and passwords)?
Oh boy. There's a list of criminal charges that could be in someone's way if that happened. Assuming Canadian law, there are a couple offences coming to mind:
Unauthorized use of a computer. §342.1 of the Criminal Code of Canada:
Everyone is guilty of an indictable offence and liable to imprisonment for a term of not more than 10 years, or is guilty of an offence punishable on summary conviction who, fraudulently and without colour of right,
(d) uses, possesses, traffics in or permits another person to have access to a computer password that would enable a person to commit an offence under paragraph (a), (b) or (c).
Mischief in relation to computer data. §430(1.1) of the Criminal Code of Canada:
Every one commits mischief who wilfully
(c) obstructs, interrupts or interferes with the lawful use of data;