WordPress.org

CommunityAPI is a lightweight Single Sign On and directory service. The following capabilities are supported:

• Single Sign On via a known identity provider supporting CommunityAPI calls.
• Password proxy support, verifying a submitted user/pass by sending them to the identity provider.
• Auto provisioning of users, should they not exist in the WordPress database.
• Authorization set by group membership in the identity provider.

The provider and consumer (wordpress) does not have to reside on the same host.

To examplify: If you have a phpBB3 forum with the CommuntiyAPI phpBB3 plugin, installing this plugin will allow you to login to wordpress using the phpBB3 credentials, will log you in automatically as long as you have a phpBB3 session going and will create the user automatically if it doesn't already exist locally in WordPress. Group memberships in phpBB can be tied to access levels in WordPress, in which case the access level of the user is modified accordingly each login.

As such, you'd really want to operate both the provider (phpBB3 or similar) and the consumer (WordPress) under the same administative 'domain', since WordPress as a whole needs to trust the provider.

Local wordpress users are supported, but keep in mind that a CommunityAPI user with the same username will be seen as the same user.