Hi Ashwin,
PrinceXML (http://www.princexml.com/) seem to be the daddy of all HTML/
CSS to PDF converters. The evaluation is free although a commercial
server license will set you back a hefty US$3800.
Here's an excellent article on integrating it with Ruby:
http://sublog.subimage.com/articles/2007/05/29/html-css-to-pdf-using-ruby-on-rails
Cheers, Olly.
And that’s for a serverwide license. If you want to install your rails app on several servers (e.g. installations on several customer servers), you’ll need to pay that license fee over and over again.
A workaround to this might be to run a PDF service on a single machine
and just make calls to that from your server cluster 
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