Product review (1 stars): I spoke too soon
We've lived for 10 years wondering what that hole in our dishwasher was.  One day a friend told us it was for something called Rinse Aid, and needed a cap.  I went to Amazon and was thrilled that such an esoteric item even existed, yet alone for just a few bucks.  This is why Amazon is so special.

Updated Feb. 20, 2015
I spoke too soon in my last review.  Turns out the cap was about a millimeter too large, and so it was useless.  It's too much trouble to return.