Besides using sp_helprotect, what are other system functions/system stored procedures/system tables/system views that I can use to find out granted permissions of a user?

I have tried to trace sp_helprotect and it seems to me that I can use sys.database_permissions.

There is of course the PERMISSIONS() function, which you can invoke only for the currently connected login, and for one object at a time. It gives you the effective permissions, although it won't tell you if a permission was denied - just that it was or was not granted. It covers column level permissions too. Check it out in BOL