Simon Willison’s Weblog

Are there any YC startups based around open source software?

My answer to Are there any YC startups based around open source software? on Quora

There have been a few.

Meteor.js is the most obvious example (YCS11, fully open source, raised $11.2 million): Meteor Raises $11.2M Round Led by Andreessen Horowitz.

Reddit have released all of their source code though I’m pretty sure they did that long after they went through YC: reddit on GitHub

DotCloud (YCS10) have pivoted to focus on Docker, which is fully open source.

If you browse through the list on Y Combinator Company List you’ll certainly find some others.