The URL ID module provides a field that gives an entity a 'global' ID, based on the domain name, entity name and entity id (such as http://example.com/node/1).
The idea is that remote entities can be downloaded, inheriting the remote URL ID – a node could have the URL ID 'http://example2.com/node/123'.
The Lazy URL ID Reference module provides a lazy-loading reference field, utilizing URL IDs. The idea is to refer to a URL ID such as 'http://example2.com/node/123'. If there is a local entity with this URL ID, the reference field will point to it (eg. node/456), if not, it will point to the remote source (eg. 'http://example2.com/node/123').
Furthermore, URL ID fields have fetchers – plugins that allow fetching remote data and saving it locally. These fetchers can also affect the reference link, for example by automatically download the referenced entity, or to redirect to the remote source.
This is a work in progress. It is related to a project for being able to download and sync help pages from drupal.org to your local site. See the following links for more information:
Node Block Auto is an extension of the excellent Node Block module, which allows developers to specify content types that are automatically provided as blocks.