It’s hard to see examples of FOSS being used widely between people. This could be the 2nd time after Mastodon that a FOSS project receives massive attention.
I can’t recall other software that the public might have used without knowing it’s open source. The ones I can get off the top of my head may be Firefox or Inkscape.
Obviously this is referencing apps rather than libraries etc.
GIMP is used a lot by people looking for a “free Photoshop” as is Libreoffice for people looking for a “free MS Office”. VLC is used by a huge number of people. OBS, Audacity, various Chromium based browsers like Brave.