I&#x27;m talking about things like client libraries for closed-source servers, since that&#x27;s the context where you&#x27;re most likely to have a library and care about what language it&#x27;s implemented in. I&#x27;m not talking about the core product itself having source available.<p>A number of software-as-a-service companies are making <i>extremely</i> successful businesses with completely proprietary servers and open libraries. Amazon AWS and Slack both come to mind without thinking very hard, but there are a lot of others.<p>If you want to reject software-as-a-service, there are a good number of companies selling proprietary (or at most &quot;open core&quot;) desktop&#x2F;server applications with open-source client libraries. VMware, a bunch of NoSQL companies, a couple of network monitoring tools, etc. also come to mind. Those sorts of companies would have given you a closed-source C library 10-30 years ago.<p>And, of course, there are people who sell hardware (graphics cards, network cards, RAIDs, etc.) who would have released proprietary tools and drivers in the past and are now releasing both as open-source because that&#x27;s a competitive advantage because their stuff works better.