External User Authentication - Are there portal options for .Net web apps and SharePoint?

Question

We have several existing web applications and SharePoint sites that we want to take off the intranet and make public to authorized external users. There are several commercial Identity and Access Management (IAM) solutions available but I'm trying to find an open source alternative.

I would like to avoid building a custom membership provider or forms based authentication. Building the sites as Portlets is not an option.

I would like to have a portal that will act like a reverse proxy Chapter 7 (specifically Fig. 7-3 on p.253 gives a good description of the "reverse proxy" functionality that I'm looking for: http://www.redbooks.ibm.com/abstracts/sg246014.html?Open

Thanks for your help!