It's not a secret that most if not all YC-funded startups are built on top of mountains of free code written by someone else. Nobody here pays for XML parsing libraries or POP3 implementations. But I am yet to see the code produced by YC-funded companies themselves.
Scribd, I want to use iPaper on my own site without using your servers. I think it will be beneficial to my customers. Since you took millions of free code for parsing all these pre-existing document formats, don't you think you owe to the community and should release your convert-to-flash code as well?
I know that GPL only covers the case of "distributing" software and running it on your own servers releases you from an obligation to give something back in return.
But does it really justify your behavior of being a parasite?
This is clearly not the only way of giving back to the community. What about all the contributions they make to the projects they use. I believe scribd has stretched rails to limits beyond any other site and has made contributions from it. Adam from heroku just discussed a patch he submitted to rails too. The list goes on, I'm sure.
Also, you sound like you have used iPaper, a free service provided by Scribd. Perhaps you should consider an amicable request for an open version of iPaper (since that is obviously why you made this post) rather than calling YC companies parasites. On a related note, I would be interested to hear what parts of your company's product you have opened.
I'll build a list:
http://blog.reddit.com/2008/06/reddit-goes-open-source.html
http://portal.insoshi.com/
http://ycombinator.com/seriesaa.html
http://www.webmin.com/index8.html
http://opensource.heroku.com/
http://trac.youos.com/
http://www.getdropbox.com/install?os=linux
http://www.xobni.com/about/opensource
http://appjet.com/app/150996096/source