I know it's not OSS, but scribd.com is a really useful service. They have a decently powerful API that lets you upload the doc and let them handle hosting, rendering, metadata, etc.

I'm not sure if this will work for you, but sites like drop.io seem to use it pretty well.

Google docs viewer has worked well for me: https://docs.google.com/viewer

Example here: http://googlesystem.blogspot.com/2009/09/embeddable-google-d...

I built this for a project I'm working on. http://github.com/Benmills/pdfize

It's a jquery plugin that uses the google pdf viewer. Here is a demo: http://files.bmdev.org/pdfize.html

I can't think of any OSS, but Vuzit makes a pretty good one that you can add and embed in your website without having to host your files with them http://vuzit.com/.

Don't know if I would call it really good, but pdf2swf might be a start.

http://www.swftools.org/

Not sure if this is an option for you, but you could run OpenOffice in headless mode and script it to convert pdf files to html.

if you are into the java ee scene you can look into icepdf by the icefaces people. I've never personally used it but is is OSS now so it might work out for you.

http://www.icepdf.org/