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.