Question re: relational technique

Here is a sample table:

item   item_id int   name    char   attrib1 char   attrib2 char   attrib3 char

One problem with the above is that the list of attributes is fixed. I am
looking for a way to assign new, previously undefined, attributes to items
without changing the table structure. Is it ever appropriate to do the
following?

item   item_id int   name    char

details   item_id         int   attribute_name  char   attribute_value char

If this is a reasonable technique are their guidelines for its use? When is it
approptiate? When not?

Thanks,
Bob