System stored procedures are useful administrative tools. Below, you may find 10 of them which I consider as the most useful:

sp_help – info on objects in database, data types
sp_spaceused – displays space on the disk allocated to database and storage for each database objects including number of rows in the table
sp_who – returns info on currently connected users and their sessions
sp_lock – returns info on locked objects * procedure will be replaced in the future by systematic view Sys.dm_trans_locks
sp_configure – returns setting and enables changes to configuration for SQL instance
sp_tables – returns list of the tables which can be queried
sp_columns – list of the columns of database objects
sp_depends – Procedure receives argument of object and returns list of objects on which is mentioned object dependent and other way around – on which are other objects dependent (dependencies)

More on sql systematic procedures can be found on Microsoft web here