Just a quick list of undocumented SPs, many of which are deprecated.
Note, use caution using some of these ! But others are very useful (e.g. sp_msforeachdb)
sp_checknames
sp_columns_rowset
sp_enumoledbdatasources
sp_fixindex
sp_gettypestring
sp_ms_marksystemobject
sp_msaddguidcolumn
sp_msaddguidindex
sp_msaddlogin_implicit_ntlogin
sp_msadduser_implicit_ntlogin
sp_mscheck_uid_owns_anything
sp_msdbuseraccess
sp_msdbuserpriv
sp_msdependencies
sp_msdrop_object
sp_msforeachdb
sp_msforeachtable
sp_msget_qualified_name
sp_msgettools_path
sp_msgetversion
sp_msguidtostr
sp_mshelpcolumns
sp_mshelpindex
sp_mshelptype
sp_msindexspace
sp_msis_pk_col
sp_mskilldb
sp_msloginmappings
sp_mstablekeys
sp_mstablerefs
sp_mstablespace
sp_msunc_to_drive
sp_msuniquecolname
sp_msuniquename
sp_msuniqueobjectname
sp_msuniquetempname
sp_tempdbspace
sp_who2
xp_delete_file
xp_dirtree
xp_enum_oledb_providers
xp_enumcodepages
xp_enumdsn
xp_enumerrorlogs
xp_enumgroups
xp_fileexist
xp_fixeddrives
xp_get_mapi_default_profile
xp_get_mapi_profiles
xp_getnetname
xp_qv
xp_readerrorlog
xp_regaddmultistring
xp_regdeletekey
xp_regdeletevalue
xp_regenumvalues
xp_regread
xp_regremovemultistring
xp_regwrite
xp_subdirs
xp_varbintohexstr
