In the Star Trek universe: The hypospray was Dr. McCoy's Vulcan neck pinch. If Spock could not render unconscious an evil alien or crewmember-gone-bad with his trademarked Vulcan grip, McCoy usually had a hypospray handy with who-knows-what cocktail of powerful sedatives. The hypospray was a needle-free device that could subcutaneously inject drugs via forced air.
In real life: On Wikipedia, the fictional hypospray is compared to the jet injector, but those are not modern enough and seem too clunky.
There are much-better, more modern versions of the hypospray either on the market or in development. Among them is the Sumavel DosePro (pictured right) from San Diego-based Zogenix. The DosePro, about the size of a fat marker, delivers the migraine drug sumatriptan subcutaneously and without needles. The force of a small amount of compressed nitrogen pushes the liquid sumatriptan through skin in less than one-tenth of a second, according to the company.
That sounds like just enough time to knock out an enraged, charging Klingon.
Another similar product is the Biojector 2000, which has been on the market since 1993. It delivers intramuscular and subcutaneous injections by forcing liquid medication through a tiny orifice that is held against the skin. A very fine, high-pressure stream of medication penetrates the skin, depositing medication in the tissue beneath. The company that makes it bills the Biojector as ideal high-risk situations, like delivering medication to a patient with HIV or hepatitis. They do not, however, mention anything about its effectiveness against Rigelian fever.
For a comprehensive overview of needle-free injection devices, take a look at this 2009 article from Genetic Engineering & Biotechnology News.