Share and update the structure (content types, vocabularies/tags, views, rules, …) for several sites
We use the domain access module, https://drupal.org/project/domain

Everything is or can be shared on the multiple sites ... we choose to have different themes for each domain and we only have a few nodes that are common to multiple sites, most of our nodes are only on one site, but the content types and taxonomies and blocks and views are shared. 

There are some current domain views filters that allow exactly the same view to be used on multiple sites but show different content on each.

It is working really well for us, our only issues are that


There are some reports that combine all domains when we would like to have them separate (e.g. ubercart)
Users have to login to each site separately
All relative path links are rewritten to the correct domain automatically but if someone for some reason requests a path on the wrong domain they are not redirected and they see an access denied message.