Jitsi Meet is a good foss choice for video conferencing in general, and includes a decent screen sharing function. You can try it out on the free to use (8x8 hosted, I think?) instance at https://meet.jit.si/

Heads up: that partocular instance now requires the host to be logged in to either a Google or Gihub account first, raising privacy concerns. that's just how that instance is configured tho, to avoid abuse. it's also self hostable and there should be other free instances available if you search for them.

edit: just noticed a few already mentioned this one (sorry, i must have scanned too quickly). fyi, Jitsi works well in combo with OBS virtual camera, allowing very precise control over exactly what audio and video you're streaming