Ask HN: Best FLOSS software for an online conference?

With the CORVID-19 outbreak, many [1] conferences [2] (to name just two) have expressed an intention to "go digital". Has anyone attended an all-online "conference" before? What kinds of software is most useful for hosting such an event?

[1] https://www.fsf.org/blogs/community/libreplanet-2020-in-person-component-canceled-but-well-see-you-online

[2] https://www.suse.com/c/reimagining-susecon-2020/