For the last 10 years the US Air Force has been flying a small reusable space plane designated the X-37B, carrying out classified missions in low Earth orbit. This of course makes it something of a magnet for crazy conspiracy theories suggesting it's a weapons platform or it's changing the weather. However it was always pitched as a reusable platform to perform extended experiments in space starting back when it was a joint project with NASA. But NASA left the project after developing new flight control sensors and algorithms, because the purpose of the final product largely replicated capabilities already offered by the Space Station.

The DoD had similar testing requirements with the extra constraint of keeping the hardware and tests classified, so, the X-37B offers them this capability.

Photos of X37B in space by Ralph Vandebergh
  / ralfvandebergh