See e.g. for a list of applications https://en.wikipedia.org/wiki/Qt_(software)#Qt_market

Of course it depends on where your interests lie but here's some examples: VLC, Spotify (Linux), old Opera, Krita, Google Earth, Tesla Model S in-car UI, webOS, SailfishOS, AMD's Radeon Software Crimson driver tool, and many others including what's already mentioned, Battle.net.