Lulu is the free and open source macOS firewall (like Little Snitch)

Objective-See is great stuff.

I threw a tiny amount of money at them via Patreon (IIRC).

BlockBlock is a thing like "Little Snitch for kernel extensions" -- if a process tries to install a kernel extension, it is blocked until you approve the action via an alert. It is in the process of being reworked for macOS Monterey, but I've used BlockBlock on macOS 10.11 up to Big Sur and it does the job.

Anything similar for Windows?

We are working on something similar, it’s in Alpha and we are looking for testers: https://safing.io/portmaster/

What are the advantages of Lulu over Little Snitch?

It’s free (it’s right in the title)

The information that little snitch is not free, is however, not in the title, it even implies it is. Probably why they asked the question.

Yeah, I see that. My intention was "it's a macOS firewall (like little snitch)", not that it was "free and open source (like little snitch)". Apologies, but I can't edit the title anymore.

No worries. I didn't object to the title, so much as to the parent comment that suggested the original question was something that should have been obvious from the title, when it was not.