Of course by mentioning any name here I risk that they might be a future candidate of closing down and you say "gotcha" :-)
Not all of them are "open core" in the sense that they have a core product and proprietary extensions (some just opensource everything they make), but a number of relevant companies around FOSS products: Gitlab, MariaDB Corporation, Blindside Networks (Big Blue Button), Qt Company, Mattermost Inc.
Not all of them are "open core" in the sense that they have a core product and proprietary extensions (some just opensource everything they make), but a number of relevant companies around FOSS products: Gitlab, MariaDB Corporation, Blindside Networks (Big Blue Button), Qt Company, Mattermost Inc.