I don't know of a 3rd party solution; I'd start with this page
http://www.codeguru.com/csharp/csharp/cs_internet/desktopapplications/article.php/c13163
to get the FTP basics.
Save the file from the FTP with a Unique ID then give them a new link that with the original file name.

Here's how I force downloads so I can rename the files to the original file names, vs the UID I saved them as.
[code]
            Response.Clear();
            Response.AddHeader("content-disposition", "attachment; filename=" + fileName);
            Response.WriteFile(virtualPath);
            Response.ContentType = "";
            Response.End();
[/code]

There may well be other better options - If I wanted to do this, that would be how I started.
This is, of course, me spending only 10 minutes thinking about it.