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.