ActionID is a single sign on provider, used to login to many applications in the progressive ecosystem. Accounts are freely available to anyone who wants to create them, and many tools (such as openVPB) allow users to begin volunteering immediately upon account creation. For more restricted access (such as to VoteBuilder or NGP), system administrators are still in control of whether users with ActionIDs can log in and have fine grained controls over feature availability and data access. Third party applications have the same options - either trust all accounts or apply additional security.
The implementation of ActionID is currently OpenID 1.1, with plans to also support OpenID Connect in the near future. Most programming languages have available libraries for OpenID interaction - see passport-openid for node.js and DotNetOpenAuth for ASP.NET, as a few examples.
For more information on the technical details of ActionID, check out the OpenID 1.1 spec.