sp_help [table]
Gives table properties. Especially useful is that it gives column lists with data types. Also shows if there is an identity on the table.
sp_helpindex [table]
Gives list of indexes on the tables
sp_fkeys [table]
Shows which tables reference [table] with a foreign key.
sp_helpconstraint [table]
Same as sp_fkeys + shows constraints (defaults, foreign keys, and primary keys) on the table.
sp_who2
Gives information on users in the current MSSQL instance