Scientific Atlanta's SARA firmware ran on all of the Explorer cable boxes they manufactured until they were purchased by Cisco Systems in 2005 and shot by Technicolor S.A. in 2015. It's quite possibly one of the most infamous pieces of software to ever run on a cable box.
Scientific Atlanta, or SciAtl, were a company founted in 1951 in Lawrenceville, Georgia (ironic) by 6 scientists from GTRI. Their set top box industry dominated the market for a while, from 2003 to 2005, and had 100% market share with several major IPTV and cable providers, such as TCI, AT&T, and Charter Communications. Eventually, they were purchased by Cisco Systems in 2005. 10 years later, they would be sold by Cisco Systems to Technicolor S.A, who would later dispose of them.
SARA cable boxes are still in use in some places today, mostly rural areas and cities with far too many people. AT&T employs them for it's IPTV service, U-verse, even today. However, it began the shift to much more modern Arris cable boxes many years ago.
SARA was reportedly very difficult to use, it's search, DVR recording, and graphics criticised for being clunky and unworkable. In my opinion, though, it's quite a bit more charming than anything we have these days.