Open Office, Libre Office, Only Office to start.
But the UX is just a lot worse, and it isn't easy to go from one application to another because they're slightly different enough that your productivity takes a hit from all the small papercuts.
I'm waiting for some FOSS spreadsheet solution that doesn't just try to copy Office, but comes up with something better. Then it'll feel like it's worth it to learn a whole new program and its UX, rather than just suffering through it because you wanna use FOSS.
When there is a significant difference, it needs to be shown what the equivalent is in the alternative. The jump can be a bit mitigated by education or information, but usually by only so much, where it's still seen as attractive.
But once you get used to those differences, (also, knowing that there are a handful of themes that can shorten the difference significantly) then it becomes a non-issue after less than 10 hours of use.
Speak for yourself. I see that LibreOffice's default UI is still a normal WIMP UI and this is a plus for me. I hated when MS Office switched to the ribbon in Office 2007.
"So you want to insert a row in a table? Great, just click on Table > Insert > Row... Oh well. Nevermind, just show me your screen and I'll hunt the functionality in that stupid ribbon."
We don't need less, but more, Office programs that respect GUI UI conventions.
It's self-hostable (and the community version is FOSS I think), and really useful in a way I find better than just a spreadsheet.
It's no good for importing complex excel things, but I've found it very useful for new work.
There's two more things excel is horrible at: choice of extension language and being able to graduate your spreadsheet into a real program. You fix the extension language by using something like web assembly on the back end, and probably bundle one or more compilers to go from $lang to web assembly in order to be user friendly. Lastly you fix the last problem by virtue of doing all of the above. The second two features won't draw in new users much, so they're less important in the short run but make it a lot more sticky.
I'm not in a place in life to put much free time into that project, and ideas are cheap ...
Looks like there was finally a OpenOffice release recently but that was after years of people complaining of security vulnerabilities not fixed in the release version.