Necklace of soul switching (Dragon 294, p77) is interesting.
You'd be right to point out that the function of this item, putting you into a new body permanently, can already be accomplished by true mind switch or by a combination of mind switch and astral seed (I think that's how that goes). Those methods, though, don't work if the body you want to get into has more hit dice than you manifester level, and they don't give you the supernatural abilities of the new body.
The necklace has its own limitations. The soul to be transferred can only be up to 20th level. The price tag comes in at a hefty 326,000 gp. It's actually dangerous for you if the target isn't willing or under your control, so it will forfeit its save. But, and I can't stress this enough, there's no hit die cap for the body, and you gain its supernatural abilities, both attacks and qualities.
One application is getting yourself into the tarrasque's body without needing any shenanigans to reduce its hit dice or massively boost your manifester level. Chronotyryn (Fiend Folio, p33) is already an option for shapechange, but this way you can walk around as a chronotyryn all day without finding a way to squeeze in persistent spell, and you don't have to worry about dispel.