User2sid and Sid2user are two small utilities for Windows NT, created by Evgenii Rudny, that allow the administrator to query the SAM to find out a SID value for a given account name and vice versa. User2sid.exe can retrieve a SID from the SAM (Security Accounts Manager) from the local or a remote machine and Sid2user.exe can then be used to retrieve the names of all the user accounts and more. These utilities do not exploit a bug but call the functions; LookupAccountName and LookupAccountSid respectively. These tools can be called against a remote machine without providing logon credentials except those needed for a null session connection. These tools rely on the ability to create a null session in order to work.
Windows Enumeration: USER2SID & SID2USER.