http://www.devarticles.com/c/a/SQL-Server/Extended-Stored-Procedures-Intro-And-10-Cool-Examples/
Authors gives good brief on 10 cool features in SQL Server repository.
xp_fileexist filename [, <file_exists INT> OUTPUT]
exec master..xp_fileexist 'c:\test.file'
xp_regread root_key, path_key, key_value
master..xp_regread 'HKEY_LOCAL_MACHINE', 'SYSTEM\Setup', 'SystemPartition'
exec master..xp_repl_encrypt @cardNumber1 output
exec master..xp_dirtree 'c:\mydir'
set @xml = '<people><person name="John Doe" age="30"/></people>'
exec sp_xml_preparedocument @id output, @xml
select * from OpenXML(@id, '/people/person')
exec master..xp_availablemedia