Unity engine , the most popular game engine based on mono. Also gnome software like glib , cairo , harfbuzz prolly used as foundation by all OSes, gui toolkit, programming language or browser on the market.
It's likely you do use Mono or a successor. The modern dotnet framework is a descendent of Mono and is used in a variety of websites, games, and other applications.
IIRC, Mono's base class library was also used to fill the cross-platform gaps in .NET Core 2-3 back when they ported the Framework APIs they had removed for Core 1. I don't know how much of that remains though.