In the late-1980s, two very interested parties were interested in buying Hanna-Barbera studios and libraries and Ruby-Spears animation libraries from The Great American Entertainment Company (formerly Taft Entertainment). One of those parties was Turner Entertainment while the other was Viacom. Viacom offered more money and eventually won the properties in 1991.
If you want to look at how much of an impact the purchase of the venerable Hanna-Barbera studios has made at Viacom, look at a pair of their popular franchises, The Flintstones and Scooby-Doo.
The Flintstones, one of the longest-running animated series in American history, is a prominent part of the Nick-at-Nite, TV Land, and Nicktoons TV lineups. The original episodes are part of the retro-laden lineup of TV Land and in rotation afternoons on Nicktoons, a channel conceived by longtime network consultant Betty Cohen after the purchase of the Hanna-Barbera library. New episodes made by Hanna-Barbera, spurred on by the launch of The Simpsons on Fox, dominate the Nick-at-Nite lineup seven nights a week. Done in the same style as the original 60s series, The Flintstones has been modernized (well, as modern as you can make a series based in the stone age) and animated by a new generation of creators, The Flintstones remains one of the most popular animated series in North America.
Scooby-Doo has remained a fan-favorite for decades throughout many incarnations. It is the only series to air new episodes on both MTV and Nickelodeon. In 1994, Nickelodeon premiered a new cycle of Scooby-Doo episodes in the tradition of the classic Scooby-Doo, Where Are You, which remains a standard on both Nickelodeon and Nicktoons. Almost a decade later, Nickelodeon's Teenick lineup and MTV shared broadcast rights to a new, more contemporary, teen-oriented Scooby-Doo series, gaining critical praise and numerous award nominations. It was also the first Scooby-Doo series not to have the popular Great Dane as the prominent name on the series, now called The Mystery Machine. The Mystery Machine is one of Nickelodeon's most popular series on the air.
Hanna-Barbera was the studio responsible for the Nicktoons project, a collection of original animated series and shorts created by individuals and outside companies. After reintroducing classic characters like Yogi Bear, Huckleberry Hound, Pixie and Dixie, and Auggie Doggie as well as Viacom-owned characters like Mighty Mouse and Deputy Dawg (including a memorable crossover short with Hanna-Barbera's Quick Draw McGraw), Hanna-Barbera introduced new franchises like The Powerpuff Girls, Rocko's Modern Life, Dexter's Laboratory, Hey Arnold, The Fairly Oddparents, Mina and the Count, Spongebob Squarepants, and Invader ZIM. Hanna-Barbera also animated MTV originals like Downtown, Space Ghost, Aeon Flux, and Samurai Jack. Hanna-Barbera also provided animation services to outside acquisitions like Klasky-Csupo's Rugrats and Wild Thornberrys and Spumco's The Ren and Stimpy Show.
Because of the reverence Viacom has for the Hanna-Barbera name, they refrain from renaming the studio Nickelodeon Animation Studios, though in essence, that's what it has become.