This ring is from LoM, pg 130. So basically it's a Reduce Person that applies to any intelligent creatures, lasting until you remove the ring or command it to restore size. For only 20,000gp it seems like a great bargain to spellcaster and sneaking characters, perhaps even better than a ring of invisibility.
I remember seeing a few builds (killer gnome?) based around being small, but I can't think of anything else to really make use of it.
What do you think? Can you think of any useful application of this lovely little ring?