How to install SQL Server management studio only?

On the download page:
http://www.microsoft.com/en-us/download/details.aspx?id=29062

Select the file just for SQL Server management studio installation:
ENU\x64\SQLManagementStudio_x64_ENU.exe

How to get list of SQL Server instances on local machine?

In command line prompt (Run as admin):
reg query "HKLM\Software\Microsoft\Microsoft SQL Server\Instance Names\SQL"