Azure ACS offers OAuth and OpenID providers, and now that ASP.NET 4.5 offers the same thing, without the added cost, is there any reason for a .NET developer to consider ACS?
(Disclaimer, I don’t know what version of oauth each service provides)
The primary benefit to using ACS is that you can integrate login systems which are not compatible with DotNetOpenAuth; namely, that you could combine external logins with an Active Directory (including both on-premises AD as well as AD in the cloud like Office 365). Your customers could use their accustomed identities from 3rd parties, and your administrative team could use their existing domain credentials.