If you want to use Windows Authentication in the Microsoft SQL Server Management Studio to connect to a Microsoft SQL Server in an other domain or with an other username, you can use the RUN AS command in Windows:
runas /noprofile /netonly /user:MyDomain\User1 "C:\Program Files (x86)\Microsoft SQL Server\100\Tools\Binn\VSShell\Common7\IDE\Ssms.exe"
This will start the Microsoft SQL Server Management Studio under the account "MyDomain\User1". You’re workstation doesn’t even have to be a member of the domain "MyDomain" as long as you know the correct username and password.
This will work even if there are no trusts between the domains.
Bas Kroes, reminded me of the Windows UI “Run as” option in Windows 7:
If you are member of the same domain or there is a trust between the domains, you can just use the Windows UI, by SHIFT + Right click on the shortcut to the Microsoft SQL Server Management Studio in the start menu en choose “Run as different user”:
If you use the UI option and, you are on a different domain with no trust, you will get the following exception:
“The security database on the server does not have a computer account for this workstation trust relationship”