The TacRack replaces the rear slide end cap of either a Glock or M&P pistol. It is CNC machined out of 6061 aluminum. The TacRack has ears that allow the user to rack the pistol on a hard surface such as a kydex holster or pistol belt.
Here is a review by Trop Guns.
The TacRack seems handy for those who do not want to rack their pistols against their optic. Some pistols have sights that allow the user to rack the slide one handed. My Sig Sauer P938 rear sight sits tall enough that I can rack it off my belt. However with the TacRack you do not have to get special sights to rack the slide one handed. Or perhaps you have a set of sights that you prefer but they do not have the proper shape to rack off a belt, then the TacRack will allow the shooter to manipulate his or her pistol one handed.
The TacRack retails for $29.99 and they even have custom laser engraving available for just a few dollars more.