<p>I've used a tool from Aspose (<a href="http://www.aspose.com/categories/file-format-components/aspose.pdf.kit-for-.net-and-java/default.aspx" rel="noreferrer">Aspose.PDF.kit</a>) to generate a JPG thumbnail on the fly of a PDF for a .NET application.  This works great,  but what kind of other alternatives are there?  Codeproject.com has <a href="http://www.codeproject.com/KB/GDI-plus/pdfthumbnail.aspx" rel="noreferrer">this tutorial</a>, but it requires you to have the full version of Acrobat.  </p>

<p>Is there an open source alternative that's geared for ASP.NET? </p>

I absolutely despise working with PDF documents.  But when I absolutely have to, I have been using
iTextSharp
.  It has quite a bit of functionality,
the API is fairly well documented
, and it is theoretically free.  Though to use it effectively it helps if you get the book that they push on their website.  I'm not certain if you can easily make thumbnails with it, but it has worked for me in generating and manipulating PDFs on every project I've worked on so far.