Well, it depends how absolutely small and in what 9mm flavor you want. The Ruger LCP/Kel-tec380 would both fit the category of super-small in your front shirt pocket/key-ring accesory gun category. stepping one step up in size and power the Khar 9mm/.40, KT PF9 and KT11/KT40 would fit in the next size up, while still being small enough to be pocket carried if you have reasonably sized pockets.
Of those, I would say either LCP or the Khar are your best choices. The Keltec's can be a mixed bag, although they have gotten much more consistent as of late. Main issue is that the browning system was never meant to be scaled down that far, so its as much a flaw of the design as the guns. Some get ones that are flawless out of the box and for thousands of rounds with no hicups to speak of, while some get ones that are lemons. Mind you I own a PF9 and I am very pleased with it.