Ask HN: what are examples of successful “open-source alternatives”?

It seems like every day there’s a Show HN post for another “open source alternative” to a popular SaaS app.

And indeed, this search shows 34 pages of results, with clones of everything from Notion to Jira to Hex to Figma to Carta.
https://hn.algolia.com/?q=open+source+alternative

I have some personal hypotheses on why this is a common pattern, but I am mostly wondering whether any of these have really succeeded as projects / businesses. Are there any examples of these “open source alternative”-type products really taking off?

This can be taken however you want – either big revenue, or big user base – I’m just looking for some marker of being a breakout hit.