This sp_change_users_login is depreciated as of SQL 2008 but still works well.  If you pass in the 'report' option then it will list all of the users that don't have an associated login.  

EXEC sp_change_users_login 'report'


If you want to run it for all of your databases you can do it like this.

EXEC sp_msforeachdb 'use [?]; PRINT ''?''; EXEC sp_change_users_login ''report'';'


If you look it up in BOL you will also find options for fixing "orphaned" users.

Find orphaned users
Could you, for example, have one of your craft skills filled in as carpenter, but have no ranks in it?  Being a very bad carpenter, but a carpenter nevertheless.
