By my count, there are more than 100 undocumented stored procedures, not counting undocumented replication stored procedures. I've listed many of them in the section that follows. I haven't included every last one of them for several reasons:
There are simply too many to cover in any sort of depth. This is why I've intentionally left out the undocumented routines relating to replication. Replication is a world unto itself.
Some undocumented routines are so fragile and add so little value to the Transact-SQL command set that they are best left undocumented.
Some of the undocumented routines behave so erratically or are so dependent on code external to SQL Server (e.g., in Enterprise Manager or SQL-DMO) that they are either ...