There's no Superman, and there's no such thing as kryptonite, either--at least not one that glows green and contains the element krypton. In the newest installment of the movie series, 2006's Superman Returns, super-villain Lex Luthor reveals the mineral's made-up chemical formula to be sodium lithium boron silicate hydroxide, but geologists unearthed something with that very structure in Serbia in 2007. However the real-life mineral isn't called kryptonite because krypton already exists--it's a noble gas and nowhere to be found in the newly found mineral, which the discoverers dubbed jadarite.
Krypton is useful, however, and is often employed in fluorescent lamps. Top-of-the-line double-insulated windows also use krypton gas rather than argon or plain old air for insulation. Just don't try to use it to neutralize superheroes.