Office 2007 has a free download that will allow you to save word
documents directly to PDF. That's what I use.

I am trying to integrate WORD->PDF converter  to
Sharepoint's document conversion service. Do you know any existing
products doing this?

Theres an article on how to do just that here

www.codeproject.com/useritems/DOC2PDF_in_MOSS.asp

We use ABCpdf in several of our applications with SharePoint, Biztalk,
and even some older com apps.

Until I got my hands on Adobe Pro, I used CutePDF.

Recommend you Nemo PDF Converter which can help you convert document files to PDF easily with text, layouts, tables and images preserved accurately. And it also supports batch and partial conversion.

There are too many options available to convert PDF, I tried almost all of them, and finally I stayed at ClassicPDF editor, i found it one of the best among all PDF softwares. Its an all in one package!
if you have a one time conversion or a small level work then you can use the trial as well!