Query to list all stored procedures

What query can return the names of all the stored procedures in a SQL Server database

If the query could exclude system stored procedures, that would be even more helpful.

As Mike stated, the best way is to use information_schema. As long as you're not in the master database, system stored procedures won't be returned.

SELECT * 
  FROM DatabaseName.INFORMATION_SCHEMA.ROUTINES
 WHERE ROUTINE_TYPE = 'PROCEDURE'

If for some reason you had non-system stored procedures in the master database, you could use the query (this will filter out MOST system stored procedures):

SELECT * 
  FROM [master].INFORMATION_SCHEMA.ROUTINES
 WHERE ROUTINE_TYPE = 'PROCEDURE' 
   AND LEFT(ROUTINE_NAME, 3) NOT IN ('sp_', 'xp_', 'ms_')

SELECT name, 
       type
  FROM dbo.sysobjects
 WHERE (type = 'P')