See below a list of OSS from YC companies.

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