This one's got a bit of a steep price, but I still think it's a bargain:
Ring of Nine Lives (MIC 126) for 45,000 gp.
As the name implies, it has nine charges, but the namesake purpose (Healing you 20 damage whenever you hit 0 or less) isn't the part I'd pay 5,000 gp a charge for. Once per day with an immediate, you can use a charge to make ANY failed save count as a success, with absolutely no limitations. Want to make a save against a deity? Surefire success!
It's a little situational, because it's usually easy enough to buff saves with superior resistance and the like, but this ring is amazing when you just absolutely need to make a save.