I'd like to move a site that currently uses the standard ASP.NET membership provider to use claims-based authentication through Windows Identity Foundation and Azure ACS.
We're going to surface the site using OAuth 2.0 secured REST services so this seems a sensible approach to follow. We also have a need to federate our authentication with external third-party ADFS and other systems, which is precisely the problem that ACS solves very well.
However, I would also like to retain the ability for our existing users to use their existing credentials.
To do this I think I need a custom STS that works with the ASP.NET Membership Provider.
All the literature (e.g. Bertocci's "Programming Windows Identity Foundation") suggests that it's a bad idea to write a custom STS. And, I agree -- I really don't like writing our own security code.
So - is there an STS available that can use Membership Provider data?
Take a look at the Identity Server (http://identityserver.codeplex.com) which uses the SQL Membership Provider.
It's a custom STS, but it's robust, extensible, and well architected.
Update: The project page is changed to http://thinktecture.github.io/Thinktecture.IdentityServer.v2/ Code has moved to https://github.com/thinktecture/Thinktecture.IdentityServer.v2