chiark-really binary package in Ubuntu Precise amd64

 really is a program that allows certain users to become whatever user
 they like on request. It is a bit like sudo in that respect.
 However, really is simpler than sudo, and doesn't give the system
 administrator any false security promises. So really is less of a
 general security risk to the system.
 .
 Unlike sudo it does not pretend that the called account can be any
 more secure than the calling account. so there is never a need for a
 password. If you wanted to restrict which commands and functions the
 called user can perform, use userv, not really or sudo.
 .
 Also unlike sudo, really only works if the calling user is supposed
 to be equivalent to root. But, really can also be used by
 root-equivalent users to become any user, not just root; in this way
 it can be a replacement for certain uses of su.

Source package