Hofmockel.org

A list of modules that promise different levels of User and Content Sharing across sites plus my 2cents.

Crossite - http://drupal.org/project/crossite
The Crossite plugin allows you to share nodes across a Drupal multi-site configuration such that certain nodes belong to certain sites. This is a form of ultra-cheap aggregation where all the data of the nodes in a Drupal multi-site is shared across all the sites. Practically speaking, when you visit the node page that belongs to a different site than the one you are current on, you will be redirect to the other site.

*Have not tested this module. Sounds interesting. The description leaves me a little confused about hove distinct the boundaries are between nodes from different sites.

Domain Access - http://drupal.org/project/domain
A node access module that enabled multiple sites to be run from one installation.
The module uses Drupal's node_access() system to determine what content is available on each site in the network. Unlike other multi-domain modules for Drupal, the Domain Access module determines user access based on the active subdomain that the user is viewing, rather than which group or site the user belongs to.

*Just learned about this module. I have not tested it but the description seems very interesting

Multiple Domains - http://drupal.org/project/multidomain
This is a module which allows you to span one site across multiple domains.

You can configure which domains to use for different parts of your site including which protocol that section uses. It can also be used to put the ecommerce or user account functionality behind https, for instance.

*Have not tested this one either. This description leaves me very confused.

Organic Groups - http://drupal.org/project/og
Enable users to create and manage their own 'groups'. Each group can have subscribers, and maintains a group home page where subscribers communicate amongst themselves. They do so by posting the usual node types: blog, story, page, etc. A block is shown on the group home page which facilitates these posts. The block also provides summary information about the group.

* Although this is not really a multisite module it can be streached to serve many multisite needs. Especially when your site is group centric rather than content centric. I love this module.

Organic Groups Sites - http://drupal.org/project/og_sites
Together, og_sites_hub and og_sites provide multisite functionality in which a central 'hub' site has a set of associated sites leveraging Organic Groups functionality to selectively share data (nodes and users), such that the hub site has access to all data while associated sites see and edit only their own data plus that specifically shared by the hub site.

*Had high hopes for this module but my tests didn't end with a functioning multisite. The concept seems flawed and not really scalable to very large number of groups. The author is not actively developing this module.