What features would you expect such a site to have?
What problems would it solve for you?
Do you have any sites in mind?
Check out TeamPlatform if your still in the hunt for a collaboration website. It will provide online 3D previews, a calendar, collaborative discussion/notes and of course file exchange. Workspace invitations are by email, and all access is via the internet. You can use it side-by-side with your PDM, as it supports assembly-part navigation and custom-properties search. -- www.teamplatform.com.
You can even publish web-pages for individual projects chock-full of upload links, download links, images/video, and 3D content for 1-way data delivery.
You might be interested in responses following this blogger's post -- http://solidsmack.com/cad-design-news/teamplatform-on-the-collaboration-scene-looking-crispy-and-clean/ -- regarding using TeamPlatform with a PDM.