Shuttle small payloads program was a big deal

As the Shuttle program comes to an end, it takes an important capability with it that is often overlooked. Over 200 small payloads flew on the Shuttle, stuffed into the nooks and crannnies left vacant by large satellites, ISS equipment, and other main payloads.
A press release from NASA Goddard recaps why these payloads mattered. It was not just science, but the opportunity for new researchers to cut their teeth on space programs.
"Between 1982 and 2003, more than 200 of these projects, including Get-Away Special (GAS) Cannisters, Hitchhikers and Spartans, flew in 108 missions. The program offered an invaluable proving ground for science and technology as well as for a large contingent of young scientists and engineers who came to Goddard in the early 1980s and grew up here working with small payloads. The Shuttle Small Payloads Project became one of NASA's most fertile nurturing grounds as well as one of NASA's most economically and technically successful programs. Many of these investigators rose to positions of authority, shaping the course of NASA science and exploration. In terms of ride-share opportunities, we know what the formula for success is and we're currently working with Marshall space flight center to ensure some funding for small mission capability on NASA's heavy-lift vehicle."

It's not clear what will replace this capability. Actually, nothing. CubeSats carried as secondary payloads are useful, but these programs allowed for human-supervised experiments and deployments, with onboard experiments returned to Earth. The Spartans were deployed, but loose, and later recaptured on the same mission after their free-flying task was done.